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This implements support for requesting a specific frame of an
image when using chrome://theme URLs. Initial support is limited
to frame 0, which is sufficient to conveniently provide stills
of animated images to Web UI.

Follow-up: Support for frame > 0 and scaling of animated images instead
of returning a scaled frame 0.

BUG=721647
TEST=Settings->People->"Change picture", only selected image is animating

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I9c920a6943cd1f1a2654716db73e8cd88575359d
Reviewed-on: https://chromium-review.googlesource.com/589709Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490623}