have canvas send discard instead of retain if the draw would overwrite everything

src/image/SkSurface_Base.h

Index: src/image/SkSurface_Base.h
diff --git a/src/image/SkSurface_Base.h b/src/image/SkSurface_Base.h
index b8eb7dd5f11d7ef92f927705d5ec27e8ff463d6f..a52a2b44d92a7be8eee31e114bc6da9a26b6c991 100644
--- a/src/image/SkSurface_Base.h
+++ b/src/image/SkSurface_Base.h
@@ -79,6 +79,11 @@ private:
     SkImage*    fCachedImage;
 
     void aboutToDraw(ContentChangeMode mode);
+
+    // Returns true if there is an outstanding image-snapshot, indicating that a call to aboutToDraw
+    // would trigger a copy-on-write.
+    bool outstandingImageSnapshot() const;
+
     friend class SkCanvas;
     friend class SkSurface;