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Currently ImageFrameGenerator can only be used for decoding from a
single thread at a time, with the access to any decoder for this
generator protected using the same mutex. This can result in janks where
the same image is being animated by multiple compositors in the same
process, since the decode tasks from these compositors can not execute
in parallel.

In order to allow multiple threads to use different decoders backed by
the same generator, this change splits the piece in the generator which
uses the decoder into a ImageDecoderWrapper. This makes it explicit that
different decoders can be used concurrently, as long as the access is
synchronized with a mutex per decoder.

The change should be a no-op.

R=chrishtr@chromium.org

Bug: 872753
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ib31ca739fbf858e0dc292dc9b47f9fd4a087956d
Reviewed-on: https://chromium-review.googlesource.com/1185629Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585740}