Prevent implicit framebuffer clear from clobbering alpha in emulated RGB

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

 /*
  * 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 969 matching lines @@
 
     // On old AMD GPUs on OS X, glColorMask doesn't work correctly for
     // multisampled renderbuffers and the alpha channel can be overwritten.
     // Clear the alpha channel of |m_fbo|.
     if (defaultBufferRequiresAlphaChannelToBePreserved() &&
         contextProvider()
             ->getCapabilities()
             .disable_multisampling_color_mask_usage) {
       m_gl->ClearColor(0, 0, 0, 1);
       m_gl->ColorMask(false, false, false, true);
+      m_gl->Clear(GL_COLOR_BUFFER_BIT);
     }
   }
 
   m_gl->BindFramebuffer(GL_FRAMEBUFFER, m_fbo);
   if (m_antiAliasingMode == ScreenSpaceAntialiasing)
     m_gl->ApplyScreenSpaceAntialiasingCHROMIUM();
   m_contentsChangeCommitted = true;
 }
 
 void DrawingBuffer::restoreFramebufferBindings() {
@@ skipping 261 matching lines @@
   if (m_pixelUnpackBufferBindingDirty)
     client->DrawingBufferClientRestorePixelUnpackBufferBinding();
 }
 
 bool DrawingBuffer::shouldUseChromiumImage() {
   return RuntimeEnabledFeatures::webGLImageChromiumEnabled() &&
          m_chromiumImageUsage == AllowChromiumImage;
 }
 
 }  // namespace blink