[Merge to 2785] Discard correct drawing buffer

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 264 matching lines @@
     if (!frontColorBufferMailbox)
         frontColorBufferMailbox = createNewMailbox(createTextureAndAllocateMemory(m_size));
 
     if (m_preserveDrawingBuffer == Discard) {
         std::swap(frontColorBufferMailbox->textureInfo, m_colorBuffer);
         attachColorBufferToReadFramebuffer();
 
         if (m_discardFramebufferSupported) {
             // Explicitly discard framebuffer to save GPU memory bandwidth for tile-based GPU arch.
             const GLenum attachments[3] = { GL_COLOR_ATTACHMENT0, GL_DEPTH_ATTACHMENT, GL_STENCIL_ATTACHMENT };
+            m_gl->BindFramebuffer(GL_FRAMEBUFFER, m_fbo);
             m_gl->DiscardFramebufferEXT(GL_FRAMEBUFFER, 3, attachments);
         }
     } else {
         m_gl->CopyTextureCHROMIUM(m_colorBuffer.textureId, frontColorBufferMailbox->textureInfo.textureId, frontColorBufferMailbox->textureInfo.parameters.internalColorFormat, GL_UNSIGNED_BYTE, GL_FALSE, GL_FALSE, GL_FALSE);
     }
 
     restoreFramebufferBindings();
     m_contentsChanged = false;
 
     m_gl->ProduceTextureDirectCHROMIUM(frontColorBufferMailbox->textureInfo.textureId, frontColorBufferMailbox->textureInfo.parameters.target, frontColorBufferMailbox->mailbox.name);
@@ skipping 794 matching lines @@
 
 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