Fix blits from multisampled renderbuffers to alpha:false WebGL 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 56f1eea05f3abd78eada2f5db471b6905ff3b8e9..b99c5e7b642828a0bb305ffe29e5fbf20c79099d 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -232,6 +232,19 @@ class PLATFORM_EXPORT DrawingBuffer
     new_mailbox_callback_ = std::move(closure);
   }
 
+  // This class helps implement correct semantics for BlitFramebuffer
+  // when the DrawingBuffer is using a CHROMIUM image for its backing
+  // store and RGB emulation is in use (basically, macOS only).
+  class PLATFORM_EXPORT ScopedRGBEmulationForBlitFramebuffer {
+   public:
+    ScopedRGBEmulationForBlitFramebuffer(DrawingBuffer*);
+    ~ScopedRGBEmulationForBlitFramebuffer();
+
+   private:
+    RefPtr<DrawingBuffer> drawing_buffer_;
+    bool doing_work_ = false;
+  };
+
  protected:  // For unittests
   DrawingBuffer(std::unique_ptr<WebGraphicsContext3DProvider>,
                 std::unique_ptr<Extensions3DUtil>,
@@ -257,6 +270,7 @@ class PLATFORM_EXPORT DrawingBuffer
   Vector<RecycledBitmap> recycled_bitmaps_;
 
  private:
+  friend class ScopedRGBEmulationForBlitFramebuffer;
   friend class ScopedStateRestorer;
   friend class ColorBuffer;
 
@@ -317,6 +331,16 @@ class PLATFORM_EXPORT DrawingBuffer
     const GLuint image_id = 0;
     std::unique_ptr<gfx::GpuMemoryBuffer> gpu_memory_buffer;
 
+    // If we're emulating an RGB back buffer using an RGBA Chromium
+    // image (essentially macOS only), then when performing
+    // BlitFramebuffer calls, we have to swap in an RGB texture in
+    // place of the RGBA texture bound to the image. The reason is
+    // that BlitFramebuffer requires the internal formats of the
+    // source and destination to match (e.g. RGB8 on both sides).
+    // There are bugs in the semantics of RGB8 textures in this
+    // situation (the alpha channel is zeroed), requiring more fixups.
+    GLuint rgb_workaround_texture_id = 0;
+
     // The mailbox used to send this buffer to the compositor.
     gpu::Mailbox mailbox;
 
@@ -428,6 +452,11 @@ class PLATFORM_EXPORT DrawingBuffer
   // The format to use when creating a multisampled renderbuffer.
   GLenum GetMultisampledRenderbufferFormat();
 
+  // Helpers to ensure correct behavior of BlitFramebuffer when using
+  // an emulated RGB CHROMIUM_image back buffer.
+  bool SetupRGBEmulationForBlitFramebuffer();
+  void CleanupRGBEmulationForBlitFramebuffer();
+
   // Weak, reset by beginDestruction.
   Client* client_ = nullptr;
 
@@ -448,9 +477,9 @@ class PLATFORM_EXPORT DrawingBuffer
 
   std::unique_ptr<WTF::Closure> new_mailbox_callback_;
 
-  // The current state restorer, which is used to track state dirtying. It is in
+  // The current state restorer, which is used to track state dirtying. It is an
   // error to dirty state shared with WebGL while there is no existing state
-  // restorer. It is also in error to instantiate two state restorers at once.
+  // restorer.
   ScopedStateRestorer* state_restorer_ = nullptr;
 
   // This is used when the user requests either a depth or stencil buffer.