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

src/image/SkSurface_Raster.cpp

Index: src/image/SkSurface_Raster.cpp
===================================================================
--- src/image/SkSurface_Raster.cpp	(revision 8710)
+++ src/image/SkSurface_Raster.cpp	(working copy)
@@ -25,7 +25,7 @@
     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:
     SkBitmap    fBitmap;
@@ -124,13 +124,18 @@
     return SkNewImageFromBitmap(fBitmap, fWeOwnThePixels);
 }
 
-void SkSurface_Raster::onCopyOnWrite() {
+void SkSurface_Raster::onCopyOnWrite(ContentChangeMode mode) {
     // are we sharing pixelrefs with the image?
-    SkASSERT(NULL !=this->getCachedImage());
+    SkASSERT(NULL != this->getCachedImage());
     if (SkBitmapImageGetPixelRef(this->getCachedImage()) == fBitmap.pixelRef()) {
         SkASSERT(fWeOwnThePixels);
-        SkBitmap prev(fBitmap);
-        prev.deepCopyTo(&fBitmap, prev.config());
+        if (kDiscard_ContentChangeMode == mode) {
+            fBitmap.setPixelRef(NULL, 0);
+            fBitmap.allocPixels();
+        } else {
+            SkBitmap prev(fBitmap);
+            prev.deepCopyTo(&fBitmap, prev.config());
+        }
         // Now fBitmap is a deep copy of itself (and therefore different from
         // what is being used by the image. Next we update the canvas to use
         // this as its backend, so we can't modify the image's pixels anymore.