Adding virtual method to SkDeferredCanvas::NotificationClient for signaling when commands are skipped due to the skip on clear optimization.

TEST=DeferredCanvas unit test
BUG=http://code.google.com/p/chromium/issues/detail?id=116840 
Review URL: https://codereview.appspot.com/6590050

git-svn-id: http://skia.googlecode.com/svn/trunk@5747 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/utils/SkDeferredCanvas.h b/include/utils/SkDeferredCanvas.h
index f5674d1..97848f1 100644
--- a/include/utils/SkDeferredCanvas.h
+++ b/include/utils/SkDeferredCanvas.h
@@ -203,6 +203,13 @@
          */
         virtual void flushedDrawCommands() {}
 
+        /**
+         *  Called after pending draw commands have been skipped, meaning
+         *  that they were optimized-out because the canvas is cleared
+         *  or completely overwritten by the command currently being recorded.
+         */
+        virtual void skippedPendingDrawCommands() {}
+
     private:
         typedef SkRefCnt INHERITED;
     };