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/src/utils/SkDeferredCanvas.cpp b/src/utils/SkDeferredCanvas.cpp
index a854d8c..eaba503 100644
--- a/src/utils/SkDeferredCanvas.cpp
+++ b/src/utils/SkDeferredCanvas.cpp
@@ -381,6 +381,9 @@
     if (!fRecordingCanvas->isDrawingToLayer() && fPipeController.hasPendingCommands()) {
         fFreshFrame = true;
         flushPendingCommands(kSilent_PlaybackMode);
+        if (fNotificationClient) {
+            fNotificationClient->skippedPendingDrawCommands();
+        }
     }
 }