Augmenting SkDeferredCanvas notification interface to signal flushes and memory alloc...

Augmenting SkDeferredCanvas notification interface to signal flushes and memory allocations
Renamed SkDeferredCanvas::DeviceContext to SkDeferredCanvas::NotificationClient

BUG=http://code.google.com/p/chromium/issues/detail?id=136828
TEST=Added coverage for new API to DeferredCanvas unit test. Added DeferredCanvas bench test to track deferred canvas overhead cost.
Review URL: https://codereview.appspot.com/6442108

git-svn-id: http://skia.googlecode.com/svn/trunk@5078 2bbb7eff-a529-9590-31e7-b0007b416f81