Avoid GPU readback in tex(Sub)Image2D(ImageBitmap)

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

Index: third_party/WebKit/Source/platform/graphics/Image.h
diff --git a/third_party/WebKit/Source/platform/graphics/Image.h b/third_party/WebKit/Source/platform/graphics/Image.h
index 841a2ea4ea92d73524c02e32082db48001097da1..db8568a96bd56a8a51f06b5f6c09824da5ce07ab 100644
--- a/third_party/WebKit/Source/platform/graphics/Image.h
+++ b/third_party/WebKit/Source/platform/graphics/Image.h
@@ -34,6 +34,7 @@
 #include "platform/graphics/ImageAnimationPolicy.h"
 #include "platform/graphics/ImageObserver.h"
 #include "platform/graphics/ImageOrientation.h"
+#include "public/platform/WebGraphicsContext3DProvider.h"
 #include "wtf/Assertions.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/PassRefPtr.h"
@@ -137,7 +138,9 @@ public:
 
     enum TileRule { StretchTile, RoundTile, SpaceTile, RepeatTile };
 
-    virtual PassRefPtr<SkImage> imageForCurrentFrame() = 0;
+    // The returned SkImage may or may not be texture-backed. If the caller needs a texture-backed
+    // image in a 3D context such as WebGL context, it must specify the desired context provider.
+    virtual PassRefPtr<SkImage> imageForCurrentFrame(WebGraphicsContext3DProvider* = nullptr) = 0;

[f(malita), 2016/06/15 12:39:32]

Looks like we only ever pass a WebGraphicsContext3DProvider argument when
calling on a StaticBitmapImage object (in
WebGLRenderingContextBase::texImageBitmapByGPU).  Rather than exposing this on
the Image interface (encumbers all subclasses), why not make it a
StaticBitmapImage method?  Then you would also have more freedom in naming -
maybe StaticBitmapImage::imageForContext3DProvider(...)?

     virtual PassRefPtr<Image> imageForDefaultFrame();
 
     virtual void drawPattern(GraphicsContext&, const FloatRect&,