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

src/image/SkSurface_Picture.cpp

Index: src/image/SkSurface_Picture.cpp
===================================================================
--- src/image/SkSurface_Picture.cpp	(revision 8710)
+++ src/image/SkSurface_Picture.cpp	(working copy)
@@ -24,7 +24,7 @@
     virtual SkImage* onNewImageSnapshot() SK_OVERRIDE;
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y,
                         const SkPaint*) SK_OVERRIDE;
-    virtual void onCopyOnWrite() SK_OVERRIDE;
+    virtual void onCopyOnWrite(ContentChangeMode) SK_OVERRIDE;
 
 private:
     SkPicture*  fPicture;
@@ -75,7 +75,7 @@
     SkImagePrivDrawPicture(canvas, fPicture, x, y, paint);
 }
 
-void SkSurface_Picture::onCopyOnWrite() {
+void SkSurface_Picture::onCopyOnWrite(ContentChangeMode /*mode*/) {
     // We always spawn a copy of the recording picture when we
     // are asked for a snapshot, so we never need to do anything here.
 }