Notify the embedder when pre-cached data images are loaded.

Images with a src="data:.." don't cause any FrameLoader callbacks
to be called.

BUG=307531

Review URL: https://codereview.chromium.org/26375007

git-svn-id: svn://svn.chromium.org/blink/trunk@160188 bbb929c8-8fbe-4397-9dbb-9b2b20218538