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

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

Index: third_party/WebKit/Source/core/html/HTMLCanvasElement.h
diff --git a/third_party/WebKit/Source/core/html/HTMLCanvasElement.h b/third_party/WebKit/Source/core/html/HTMLCanvasElement.h
index 74346e8fb519732fe0e55d786f2ac412f3a9888b..b6e8c86207ce04b02a7b1ad40d0d17cce7e3add3 100644
--- a/third_party/WebKit/Source/core/html/HTMLCanvasElement.h
+++ b/third_party/WebKit/Source/core/html/HTMLCanvasElement.h
@@ -222,7 +222,7 @@ private:
     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;
@@ -230,7 +230,8 @@ private:
 
     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);