Make disabling 2d canvas GPU acceleration for getImageData less aggressive.

third_party/WebKit/Source/platform/graphics/ImageBuffer.h

 /*
  * Copyright (C) 2006 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 178 matching lines @@
  private:
   enum SnapshotState {
     InitialSnapshotState,
     DidAcquireSnapshot,
     DrawnToAfterSnapshot,
   };
   mutable SnapshotState m_snapshotState;
   std::unique_ptr<ImageBufferSurface> m_surface;
   ImageBufferClient* m_client;
 
+  mutable bool m_gpuReadbackInvokedInCurrentFrame;
+  int m_gpuReadbackSuccessiveFrames;
+
   mutable intptr_t m_gpuMemoryUsage;
   static intptr_t s_globalGPUMemoryUsage;
   static unsigned s_globalAcceleratedImageBufferCount;
 };
 
 struct ImageDataBuffer {
   STACK_ALLOCATED();
   ImageDataBuffer(const IntSize& size, const unsigned char* data)
       : m_data(data), m_size(size) {}
   String PLATFORM_EXPORT toDataURL(const String& mimeType,
                                    const double& quality) const;
   bool PLATFORM_EXPORT encodeImage(const String& mimeType,
                                    const double& quality,
                                    Vector<unsigned char>* encodedImage) const;
   const unsigned char* pixels() const { return m_data; }
   const IntSize& size() const { return m_size; }
   int height() const { return m_size.height(); }
   int width() const { return m_size.width(); }
 
   const unsigned char* m_data;
   const IntSize m_size;
 };
 
 }  // namespace blink
 
 #endif  // ImageBuffer_h