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

src/image/SkSurface_Base.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSurface_Base_DEFINED
 #define SkSurface_Base_DEFINED
 
@@ skipping 30 matching lines @@
      *      image->draw(canvas, ...);
      *      image->unref();
      *  }
      */
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*);
 
     /**
      *  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();
 
     // called by SkSurface to compute a new genID
     uint32_t newGenerationID();
 
 private:
     SkCanvas*   fCachedCanvas;
     SkImage*    fCachedImage;
 
-    void aboutToDraw();
+    void aboutToDraw(ContentChangeMode mode);
     friend class SkCanvas;
     friend class SkSurface;
 
     inline void installIntoCanvasForDirtyNotification();
 
     typedef SkSurface INHERITED;
 };
 
 #endif