Index: compositing_iosurface_layer_mac.mm
===================================================================
--- compositing_iosurface_layer_mac.mm	(revision 251746)
+++ compositing_iosurface_layer_mac.mm	(working copy)
@@ -119,7 +119,7 @@
       (context_ && context_->cgl_context() != glContext) ||
       !renderWidgetHostView_ ||
       !renderWidgetHostView_->compositing_iosurface_) {
-    glClearColor(1, 1, 1, 1);
+    glClearColor(0, 0, 0, 0);
     glClear(GL_COLOR_BUFFER_BIT);
     return;
   }
Index: compositing_iosurface_mac.mm
===================================================================
--- compositing_iosurface_mac.mm	(revision 251746)
+++ compositing_iosurface_mac.mm	(working copy)
@@ -360,7 +360,7 @@
     glUseProgram(0); CHECK_AND_SAVE_GL_ERROR();
   } else {
     // Should match the clear color of RenderWidgetHostViewMac.
-    glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
+    glClearColor(0, 0, 0, 0);
     glClear(GL_COLOR_BUFFER_BIT);
   }
 
Index: compositing_iosurface_shader_programs_mac.cc
===================================================================
--- compositing_iosurface_shader_programs_mac.cc	(revision 251746)
+++ compositing_iosurface_shader_programs_mac.cc	(working copy)
@@ -11,6 +11,7 @@
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/strings/stringprintf.h"
 #include "base/values.h"
 #include "content/browser/gpu/gpu_data_manager_impl.h"
 #include "gpu/config/gpu_driver_bug_workaround_type.h"
@@ -51,6 +52,7 @@
     }
 );
 
+float bgcolor[] = {1.0f, 1.0f, 1.0f};
 
 // Only calculates position.
 const char kvsSolidWhite[] = GLSL_PROGRAM_AS_STRING(
@@ -238,6 +240,16 @@
   const GLuint shader = glCreateShader(shader_type);
   DCHECK_NE(shader, 0u);
 
+  // Support customization of the background color.
+  std::string bg_shader;
+  if (shader_program == SHADER_PROGRAM_SOLID_WHITE &&
+      shader_type == GL_FRAGMENT_SHADER) {
+    bg_shader = base::StringPrintf(
+        "void main() { gl_FragColor = vec4(%f, %f, %f, 1.0); }",
+        bgcolor[0], bgcolor[1], bgcolor[2]);
+    kFragmentShaderSourceCodeMap[shader_program] = bg_shader.c_str();
+  }
+
   // Select and compile the shader program source code.
   if (shader_type == GL_VERTEX_SHADER) {
     const GLchar* source_snippets[] = {
@@ -412,6 +424,14 @@
   Reset();
 }
 
+// static
+void CompositingIOSurfaceShaderPrograms::SetBackgroundColor(
+    float r, float g, float b) {
+  bgcolor[0] = r;
+  bgcolor[1] = g;
+  bgcolor[2] = b;
+}
+
 GLuint CompositingIOSurfaceShaderPrograms::GetShaderProgram(int which) {
   if (shader_programs_[which] == 0u) {
     shader_programs_[which] =
Index: compositing_iosurface_shader_programs_mac.h
===================================================================
--- compositing_iosurface_shader_programs_mac.h	(revision 251746)
+++ compositing_iosurface_shader_programs_mac.h	(working copy)
@@ -48,6 +48,8 @@
     return rgb_to_yv12_output_format_;
   }
 
+  static void SetBackgroundColor(float r, float g, float b);
+
  protected:
   FRIEND_TEST_ALL_PREFIXES(CompositingIOSurfaceTransformerTest,
                            TransformsRGBToYV12);
Index: render_widget_host_view_mac.mm
===================================================================
--- render_widget_host_view_mac.mm	(revision 251746)
+++ render_widget_host_view_mac.mm	(working copy)
@@ -483,7 +483,7 @@
   software_layer_.reset([[CALayer alloc] init]);
   if (!software_layer_)
     LOG(ERROR) << "Failed to create CALayer for software rendering";
-  [software_layer_ setBackgroundColor:CGColorGetConstantColor(kCGColorWhite)];
+  [software_layer_ setBackgroundColor:CGColorGetConstantColor(kCGColorClear)];
   [software_layer_ setDelegate:cocoa_view_];
   [software_layer_ setContentsGravity:kCAGravityTopLeft];
   [software_layer_ setFrame:NSRectToCGRect([cocoa_view_ bounds])];
@@ -2856,7 +2856,7 @@
 
     NSRect r = [self flipRectToNSRect:gfx::Rect(x, y, width, height)];
     CGContextSetFillColorWithColor(context,
-                                   CGColorGetConstantColor(kCGColorWhite));
+                                   CGColorGetConstantColor(kCGColorClear));
     CGContextFillRect(context, NSRectToCGRect(r));
   }
   if (damagedRect.bottom() > rect.bottom()) {
@@ -2878,7 +2878,7 @@
 
     NSRect r = [self flipRectToNSRect:gfx::Rect(x, y, width, height)];
     CGContextSetFillColorWithColor(context,
-                                   CGColorGetConstantColor(kCGColorWhite));
+                                   CGColorGetConstantColor(kCGColorClear));
     CGContextFillRect(context, NSRectToCGRect(r));
   }
 }
@@ -3037,7 +3037,7 @@
     }
   } else {
     CGContextSetFillColorWithColor(context,
-                                   CGColorGetConstantColor(kCGColorWhite));
+                                   CGColorGetConstantColor(kCGColorClear));
     CGContextFillRect(context, dirtyRect);
     if (renderWidgetHostView_->whiteout_start_time_.is_null())
       renderWidgetHostView_->whiteout_start_time_ = base::TimeTicks::Now();
@@ -4045,7 +4045,7 @@
   if (!renderWidgetHostView_->render_widget_host_ ||
       renderWidgetHostView_->render_widget_host_->is_hidden()) {
     CGContextSetFillColorWithColor(context,
-                                   CGColorGetConstantColor(kCGColorWhite));
+                                   CGColorGetConstantColor(kCGColorClear));
     CGContextFillRect(context, clipRect);
     return;
   }