Implementation of preserveDrawingBuffer:Discard in software

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 306 matching lines @@
   outMailbox->set_color_space(m_colorSpace);
 
   // This holds a ref on the DrawingBuffer that will keep it alive until the
   // mailbox is released (and while the release callback is running). It also
   // owns the SharedBitmap.
   auto func = WTF::bind(&DrawingBuffer::mailboxReleasedSoftware,
                         RefPtr<DrawingBuffer>(this),
                         WTF::passed(std::move(bitmap)), m_size);
   *outReleaseCallback =
       cc::SingleReleaseCallback::Create(convertToBaseCallback(std::move(func)));
+
+  if (m_preserveDrawingBuffer == Discard) {
+    setBufferClearNeeded(true);
+  }
+
   return true;
 }
 
 bool DrawingBuffer::finishPrepareTextureMailboxGpu(
     cc::TextureMailbox* outMailbox,
     std::unique_ptr<cc::SingleReleaseCallback>* outReleaseCallback) {
   DCHECK(m_stateRestorer);
   if (m_webGLVersion > WebGL1) {
     m_stateRestorer->setPixelUnpackBufferBindingDirty();
     m_gl->BindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
@@ skipping 959 matching lines @@
   if (m_pixelUnpackBufferBindingDirty)
     client->DrawingBufferClientRestorePixelUnpackBufferBinding();
 }
 
 bool DrawingBuffer::shouldUseChromiumImage() {
   return RuntimeEnabledFeatures::webGLImageChromiumEnabled() &&
          m_chromiumImageUsage == AllowChromiumImage;
 }
 
 }  // namespace blink