Preserve alpha channel of 1.0 when emulating RGB back buffer.

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

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 167 matching lines @@
   // Whether the target for draw operations has format GL_RGBA, but is
   // emulating format GL_RGB. When the target's storage is first
   // allocated, its alpha channel must be cleared to 1. All future drawing
   // operations must use a color mask with alpha=GL_FALSE.
   bool RequiresAlphaChannelToBePreserved();
 
   // Similar to requiresAlphaChannelToBePreserved(), but always targets the
   // 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();
   WebGraphicsContext3DProvider* ContextProvider();
 
   // cc::TextureLayerClient implementation.
   bool PrepareTextureMailbox(cc::TextureMailbox* out_mailbox,
                              std::unique_ptr<cc::SingleReleaseCallback>*
                                  out_release_callback) override;
 
@@ skipping 282 matching lines @@
 
   // True if our contents have been modified since the last presentation of this
   // buffer.
   bool contents_changed_ = true;
 
   // True if resolveIfNeeded() has been called since the last time
   // markContentsChanged() had been called.
   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_;
 
   // The color space of this buffer. All buffers are assumed to be sRGB until
   // a mechanism for creating otherwise is exposed to the web.
   const gfx::ColorSpace color_space_;
 
   enum AntialiasingMode {
     kNone,
@@ skipping 28 matching lines @@
   // RuntimeEnabledFeatures::webGLImageChromiumEnabled() call with
   // shouldUseChromiumImage() calls, and set m_chromiumImageUsage to
   // DisallowChromiumImage in the case of OffscreenCanvas.
   ChromiumImageUsage chromium_image_usage_;
   bool ShouldUseChromiumImage();
 };
 
 }  // namespace blink
 
 #endif  // DrawingBuffer_h