Reland of Allow canvases to be GPU-accelerated in Workers

third_party/WebKit/Source/platform/graphics/gpu/AcceleratedImageBufferSurface.h

 /*
  * Copyright (c) 2013, 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 28 matching lines @@
 
 namespace blink {
 
 class PLATFORM_EXPORT AcceleratedImageBufferSurface : public ImageBufferSurface {
     WTF_MAKE_NONCOPYABLE(AcceleratedImageBufferSurface); USING_FAST_MALLOC(AcceleratedImageBufferSurface);
 public:
     AcceleratedImageBufferSurface(const IntSize&, OpacityMode = NonOpaque, sk_sp<SkColorSpace> = nullptr);
     ~AcceleratedImageBufferSurface() override { }
 
     SkCanvas* canvas() override { return m_surface ? m_surface->getCanvas() : nullptr; }
-    bool isValid() const override { return m_surface; }
+    bool isValid() const override;
     bool isAccelerated() const override { return true; }
     sk_sp<SkImage> newImageSnapshot(AccelerationHint, SnapshotReason) override;
     GLuint getBackingTextureHandleForOverwrite() override;
 
 private:
-    std::unique_ptr<WebGraphicsContext3DProvider> m_contextProvider;
+    unsigned m_contextId;
     sk_sp<SkSurface> m_surface; // Uses m_contextProvider.
 };
 
 
 } // namespace blink
 
 #endif