Add support for shared texture and external BeginFrame in OSR mode (issue #1006)

- Add CefWindowInfo::shared_texture_enabled and
  CefRenderHandler::OnAcceleratedPaint for shared texture support. Currently
  only supported on Windows (D3D11).
- Add CefWindowInfo::external_begin_frame_enabled and
  CefBrowserHost::SendExternalBeginFrame for external begin frame support.

libcef/browser/browser_platform_delegate.h

@@ -139,6 +139,9 @@ class CefBrowserPlatformDelegate {
   // enable transparency.
   virtual SkColor GetBackgroundColor() const = 0;
 
+  virtual bool CanUseSharedTexture() const = 0;
+  virtual bool CanUseExternalBeginFrame() const = 0;
+
   // Notify the window that it was resized.
   virtual void SynchronizeVisualProperties() = 0;
 
@@ -227,6 +230,8 @@ class CefBrowserPlatformDelegate {
   // Invalidate the view. Only used with windowless rendering.
   virtual void Invalidate(cef_paint_element_type_t type);
 
+  virtual void SendExternalBeginFrame();
+
   // Set the windowless frame rate. Only used with windowless rendering.
   virtual void SetWindowlessFrameRate(int frame_rate);