Preserve alpha channel of 1.0 when emulating RGB back buffer.

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
index 16e778c1f44c1eeb9fa3e3b0720ee17a1eb2b19e..3608f7be13cebe26fc1d8c398df63e0659765b92 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -185,6 +185,11 @@ class PLATFORM_EXPORT DrawingBuffer
   // default framebuffer.
   bool DefaultBufferRequiresAlphaChannelToBePreserved();
 
+  // Tells the DrawingBuffer that BlitFramebuffer was used to populate
+  // it this frame. This requires more expensive emulation in some
+  // circumstances.
+  void MarkBlitFramebufferUsedThisFrame();
+
   WebLayer* PlatformLayer();
 
   gpu::gles2::GLES2Interface* ContextGL();
@@ -487,6 +492,10 @@ class PLATFORM_EXPORT DrawingBuffer
   bool contents_change_resolved_ = false;
   bool buffer_clear_needed_ = false;
 
+  // True if BlitFramebuffer was used to populate the DrawingBuffer
+  // this past frame.
+  bool blit_framebuffer_used_this_frame_ = false;
+
   // Whether the client wants a depth or stencil buffer.
   const bool want_depth_;
   const bool want_stencil_;