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/native/browser_platform_delegate_native_linux.cc

@@ -41,7 +41,10 @@ long GetSystemUptime() {
 CefBrowserPlatformDelegateNativeLinux::CefBrowserPlatformDelegateNativeLinux(
     const CefWindowInfo& window_info,
     SkColor background_color)
-    : CefBrowserPlatformDelegateNative(window_info, background_color),
+    : CefBrowserPlatformDelegateNative(window_info,
+                                       background_color,
+                                       false,
+                                       false),
       host_window_created_(false),
       window_widget_(nullptr),
       window_x11_(nullptr) {}