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

src/image/SkSurface.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSurface_Base.h"
 #include "SkImagePriv.h"
 #include "SkCanvas.h"
@@ skipping 41 matching lines @@
 }
 
 SkImage* SkSurface_Base::getCachedImage() {
     if (NULL == fCachedImage) {
         fCachedImage = this->onNewImageSnapshot();
         this->installIntoCanvasForDirtyNotification();
     }
     return fCachedImage;
 }
 
-void SkSurface_Base::aboutToDraw() {
+void SkSurface_Base::aboutToDraw(ContentChangeMode mode) {
     this->dirtyGenerationID();
 
     if (NULL != fCachedCanvas) {
         SkASSERT(fCachedCanvas->getSurfaceBase() == this || \
                  NULL == fCachedCanvas->getSurfaceBase());
         fCachedCanvas->setSurfaceBase(NULL);
     }
 
     if (NULL != fCachedImage) {
         // the surface may need to fork its backend, if its sharing it with
         // the cached image. Note: we only call if there is an outstanding owner
         // on the image (besides us).
         if (fCachedImage->getRefCnt() > 1) {
-            this->onCopyOnWrite();
+            this->onCopyOnWrite(mode);
         }
 
         // regardless of copy-on-write, we must drop our cached image now, so
         // that the next request will get our new contents.
         fCachedImage->unref();
         fCachedImage = NULL;
     }
 }
 
 uint32_t SkSurface_Base::newGenerationID() {
@@ skipping 15 matching lines @@
     fGenerationID = 0;
 }
 
 uint32_t SkSurface::generationID() {
     if (0 == fGenerationID) {
         fGenerationID = asSB(this)->newGenerationID();
     }
     return fGenerationID;
 }
 
-void SkSurface::notifyContentChanged() {
-    asSB(this)->aboutToDraw();
+void SkSurface::notifyContentWillChange(ContentChangeMode mode) {
+    asSB(this)->aboutToDraw(mode);
 }
 
 SkCanvas* SkSurface::getCanvas() {
     return asSB(this)->getCachedCanvas();
 }
 
 SkImage* SkSurface::newImageSnapshot() {
     SkImage* image = asSB(this)->getCachedImage();
     SkSafeRef(image);   // the caller will call unref() to balance this
     return image;
 }
 
 SkSurface* SkSurface::newSurface(const SkImage::Info& info) {
     return asSB(this)->onNewSurface(info);
 }
 
 void SkSurface::draw(SkCanvas* canvas, SkScalar x, SkScalar y,
                      const SkPaint* paint) {
     return asSB(this)->onDraw(canvas, x, y, paint);
 }