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Image changed notifications are used by animated images to notify
LayoutObject clients that they need to repaint. These notifications
typically result in paint invalidations. Animated bitmap images have
some logic[1] to handle "falling behind" which would synchronously
fire image changed notifications during paint. This results in missed
paint invalidations as well as a changing layout tree out from under
the paint system.

This patch moves the synchronous image change notifications to an
immediate task which occurs after paint has completed.

[1] When painting animated gifs on a heavily loaded system (or a debug
build), pauses in the system can cause the animation to get behind.
When this happens, we want to advance the animation and catch-up but
prevent the next frame from using the same catch-up logic which could
get us in an infinite catch-up loop.

BUG=616700

Review-Url: https://codereview.chromium.org/2038243002
Cr-Commit-Position: refs/heads/master@{#398147}