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
3 files changed