Avoid making the shared main thread context if we won't use it.

third_party/WebKit/Source/core/html/HTMLCanvasElement.h

 /*
  * Copyright (C) 2004, 2006, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * 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 204 matching lines @@
     void didMoveToNewDocument(Document& oldDocument) override;
 
 private:
     explicit HTMLCanvasElement(Document&);
     void dispose();
 
     using ContextFactoryVector = Vector<std::unique_ptr<CanvasRenderingContextFactory>>;
     static ContextFactoryVector& renderingContextFactories();
     static CanvasRenderingContextFactory* getRenderingContextFactory(int);
 
-    bool shouldAccelerate(const IntSize&, const WebGraphicsContext3DProvider* sharedMainThreadContextProvider) const;
+    bool shouldAccelerate(const IntSize&) const;
 
     void parseAttribute(const QualifiedName&, const AtomicString&, const AtomicString&) override;
     LayoutObject* createLayoutObject(const ComputedStyle&) override;
     bool areAuthorShadowsAllowed() const override { return false; }
 
     void reset();
 
-    std::unique_ptr<ImageBufferSurface> createImageBufferSurface(const IntSize& deviceSize, int* msaaSampleCount, sk_sp<SkColorSpace>);
+    std::unique_ptr<ImageBufferSurface> createAcceleratedImageBufferSurface(const IntSize& deviceSize, OpacityMode, sk_sp<SkColorSpace>, int* msaaSampleCount);
+    std::unique_ptr<ImageBufferSurface> createSoftwareImageBufferSurface(const IntSize& deviceSize, OpacityMode, sk_sp<SkColorSpace>);

[Justin Novosad, 2016/08/26 14:11:12]

For consistency with surrounding code, please call this "Unaccelerated" instead
of "Software"

[danakj, 2016/08/26 18:28:48]

Done.

     void createImageBuffer();
     void createImageBufferInternal(std::unique_ptr<ImageBufferSurface> externalSurface);
     bool shouldUseDisplayList(const IntSize& deviceSize);
 
     void setSurfaceSize(const IntSize&);
 
     bool paintsIntoCanvasBuffer() const;
 
     ImageData* toImageData(SourceDrawingBuffer, SnapshotReason) const;
 
@@ skipping 23 matching lines @@
     // Used for OffscreenCanvas that controls this HTML canvas element
     std::unique_ptr<CanvasSurfaceLayerBridge> m_surfaceLayerBridge;
 
     int m_numFramesSinceLastRenderingModeSwitch;
     bool m_pendingRenderingModeSwitch;
 };
 
 } // namespace blink
 
 #endif // HTMLCanvasElement_h