WebGL: clarify which Front or Back buffer is used by each API.

Source/core/html/HTMLCanvasElement.h

Index: Source/core/html/HTMLCanvasElement.h
diff --git a/Source/core/html/HTMLCanvasElement.h b/Source/core/html/HTMLCanvasElement.h
index feddebbe1f98272481b54b6ce5f78f9333edb9ea..c0e1f56bd9ecfb5a661d50dba3a2ccc11ef1662b 100644
--- a/Source/core/html/HTMLCanvasElement.h
+++ b/Source/core/html/HTMLCanvasElement.h
@@ -119,9 +119,9 @@ public:
 
     void ensureUnacceleratedImageBuffer();
     ImageBuffer* buffer() const;
-    Image* copiedImage() const;
+    enum SourceBuffer { Front, Back };

[Ken Russell (switch to Gerrit), 2014/11/26 00:49:10]

There are now three copies of this enum. Could you please refactor it either
into a new header in Source/platform/graphics/ or find another one in that
directory where it could live? HTMLCanvasElement, CanvasRenderingContext and
DrawingBuffer are all allowed to access code in platform/graphics/.

[dshwang, 2014/11/26 13:37:29]

Nice suggestion. I thought it's not pretty but I don't know why I didn't
refactor it.
I move this enum to GraphicsTypes3D.h because this enum is related to WebGL and
GraphicsTypes.h includes all enums related to canvas 2d.

I change the name to SourceDrawingBuffer because there is already
blink::SourceBuffer class for web audio.

+    Image* copiedImage(SourceBuffer) const;
     void clearCopiedImage();
-    PassRefPtrWillBeRawPtr<ImageData> getImageData() const;
 
     SecurityOrigin* securityOrigin() const;
     bool originClean() const { return m_originClean; }
@@ -187,7 +187,7 @@ private:
 
     void updateExternallyAllocatedMemory() const;
 
-    String toDataURLInternal(const String& mimeType, const double* quality, bool isSaving = false) const;
+    String toDataURLInternal(const String& mimeType, const double* quality, SourceBuffer) const;
 
     WillBeHeapHashSet<RawPtrWillBeWeakMember<CanvasObserver>> m_observers;