Use GL_RGBA renderbuffer for antialiased, no-alpha DrawingBuffers.

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 321 matching lines @@
     TextureParameters parameters;
     parameters.target = GC3D_TEXTURE_RECTANGLE_ARB;
 
     if (m_wantAlphaChannel) {
         parameters.creationInternalColorFormat = GL_RGBA;
         parameters.internalColorFormat = GL_RGBA;
     } else if (contextProvider()->getCapabilities().chromium_image_rgb_emulation) {
         parameters.creationInternalColorFormat = GL_RGB;
         parameters.internalColorFormat = GL_RGBA;
     } else {
-        parameters.creationInternalColorFormat = GL_RGB;
-        parameters.internalColorFormat = GL_RGB;
+        GLenum format = defaultBufferRequiresAlphaChannelToBePreserved() ? GL_RGBA : GL_RGB;
+        parameters.creationInternalColorFormat = format;
+        parameters.internalColorFormat = format;
     }
 
     // Unused when CHROMIUM_image is being used.
     parameters.colorFormat = 0;
     return parameters;
 #else
     return defaultTextureParameters();
 #endif
 }
 
 DrawingBuffer::TextureParameters DrawingBuffer::defaultTextureParameters()
 {
     TextureParameters parameters;
     parameters.target = GL_TEXTURE_2D;
     if (m_wantAlphaChannel) {
         parameters.internalColorFormat = GL_RGBA;
         parameters.creationInternalColorFormat = GL_RGBA;
         parameters.colorFormat = GL_RGBA;
     } else {
-        parameters.internalColorFormat = GL_RGB;
-        parameters.creationInternalColorFormat = GL_RGB;
-        parameters.colorFormat = GL_RGB;
+        GLenum format = defaultBufferRequiresAlphaChannelToBePreserved() ? GL_RGBA : GL_RGB;
+        parameters.creationInternalColorFormat = format;
+        parameters.internalColorFormat = format;
+        parameters.colorFormat = format;
     }
     return parameters;
 }
 
 void DrawingBuffer::mailboxReleasedWithoutRecycling(const WebExternalTextureMailbox& mailbox)
 {
     ASSERT(m_textureMailboxes.size());
     // Ensure not to call the destructor until deleteMailbox() is completed.
     RefPtr<DrawingBuffer> self = this;
     deleteMailbox(mailbox);
@@ skipping 668 matching lines @@
     return m_wantDepth || m_wantStencil;
 }
 
 GLenum DrawingBuffer::getMultisampledRenderbufferFormat()
 {
     DCHECK(wantExplicitResolve());
     if (m_wantAlphaChannel)
         return GL_RGBA8_OES;
     if (m_colorBuffer.imageId && contextProvider()->getCapabilities().chromium_image_rgb_emulation)
         return GL_RGBA8_OES;
+    if (contextProvider()->getCapabilities().disable_webgl_rgb_multisampling_usage)

[Ken Russell (switch to Gerrit), 2016/05/19 00:06:41]

It seems a little strange that this checks a different condition than the other
code (i.e., defaultBufferRequiresAlphaChannelToBePreserved). Are you sure all of
the possible legal combinations work?

[erikchen, 2016/06/02 00:14:06]

defaultBufferRequiresAlphaChannelToBePreserved calls this method, which is
expected and correct, now that I removed the fallback code. 

[Ken Russell (switch to Gerrit), 2016/06/02 00:46:10]

Ah, I see. Thanks for the explanation.

+        return GL_RGBA8_OES;
     return GL_RGB8_OES;
 }
 
 void DrawingBuffer::restoreTextureBindings()
 {
     // This class potentially modifies the bindings for GL_TEXTURE_2D and
     // GL_TEXTURE_RECTANGLE. Only GL_TEXTURE_2D needs to be restored since
     // the public interface for WebGL does not support GL_TEXTURE_RECTANGLE.
     m_gl->BindTexture(GL_TEXTURE_2D, m_texture2DBinding);
 }
 
 } // namespace blink