Adding optimization to avoid image copy in SkSurface copy on write when content is discardable

src/image/SkSurface_Base.h

Index: src/image/SkSurface_Base.h
===================================================================
--- src/image/SkSurface_Base.h	(revision 8708)
+++ src/image/SkSurface_Base.h	(working copy)
@@ -48,10 +48,8 @@
      *  If the surface is about to change, we call this so that our subclass
      *  can optionally fork their backend (copy-on-write) in case it was
      *  being shared with the cachedImage.
-     *
-     *  The default implementation does nothing.
      */
-    virtual void onCopyOnWrite() = 0;
+    virtual void onCopyOnWrite(ContentChangeMode) = 0;
 
     inline SkCanvas* getCachedCanvas();
     inline SkImage* getCachedImage();
@@ -63,7 +61,7 @@
     SkCanvas*   fCachedCanvas;
     SkImage*    fCachedImage;
 
-    void aboutToDraw();
+    void aboutToDraw(ContentChangeMode mode);
     friend class SkCanvas;
     friend class SkSurface;