WebGL: optimize webgl.texSubImage2D(video) path. (Blink side)

Source/platform/graphics/ImageBuffer.h

Index: Source/platform/graphics/ImageBuffer.h
diff --git a/Source/platform/graphics/ImageBuffer.h b/Source/platform/graphics/ImageBuffer.h
index 30ef58220895362c18aa865202fdd2b1d399e1f4..57274d44a8d63d411a18f8b0c8937e084da2b40c 100644
--- a/Source/platform/graphics/ImageBuffer.h
+++ b/Source/platform/graphics/ImageBuffer.h
@@ -121,7 +121,9 @@ public:
     // with textures that are RGB or RGBA format, UNSIGNED_BYTE type and level 0, as specified in
     // Extensions3D::canUseCopyTextureCHROMIUM().
     // Destroys the TEXTURE_2D binding for the active texture unit of the passed context
-    bool copyToPlatformTexture(WebGraphicsContext3D*, Platform3DObject, GLenum, GLenum, GLint, bool, bool);
+    bool copyToPlatformTexture(WebGraphicsContext3D*, GLenum, Platform3DObject, GLenum, GLenum, GLint, bool, bool);
+    bool copySubToPlatformTexture(WebGraphicsContext3D*, GLenum, Platform3DObject, GLint, GLint xoffset,
+        GLint yoffset, GLsizei width, GLsizei height, bool, bool);
 
     bool copyRenderingResultsFromDrawingBuffer(DrawingBuffer*, SourceDrawingBuffer);
 
@@ -147,6 +149,10 @@ private:
         DrawnToAfterSnapshot,
     };
     mutable SnapshotState m_snapshotState;
+
+    bool copyToPlatformTextureInternal(bool isFullCopy, WebGraphicsContext3D*, GLenum, Platform3DObject, GLenum, GLenum,
+        GLint, GLint xoffset, GLint yoffset, GLsizei width, GLsizei height, bool, bool);
+
     OwnPtr<ImageBufferSurface> m_surface;
     ImageBufferClient* m_client;
 };