Deferred image decoding to handle broken GIFs

With some broken animated images the decoder could report a smaller
frame number as more data is received. We should handle this edge case
by early out. The animation sequence might not be correct. That's okay
because it's a broken file.

BUG=352421

Review URL: https://codereview.chromium.org/205333004

git-svn-id: svn://svn.chromium.org/blink/trunk@169698 bbb929c8-8fbe-4397-9dbb-9b2b20218538