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

src/image/SkSurface.cpp

Index: src/image/SkSurface.cpp
===================================================================
--- src/image/SkSurface.cpp	(revision 8708)
+++ src/image/SkSurface.cpp	(working copy)
@@ -59,7 +59,7 @@
     return fCachedImage;
 }
 
-void SkSurface_Base::aboutToDraw() {
+void SkSurface_Base::aboutToDraw(ContentChangeMode mode) {
     this->dirtyGenerationID();
 
     if (NULL != fCachedCanvas) {
@@ -73,7 +73,7 @@
         // 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
@@ -109,8 +109,8 @@
     return fGenerationID;
 }
 
-void SkSurface::notifyContentChanged() {
-    asSB(this)->aboutToDraw();
+void SkSurface::notifyContentWillChange(ContentChangeMode mode) {
+    asSB(this)->aboutToDraw(mode);
 }
 
 SkCanvas* SkSurface::getCanvas() {