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Allows a preferred and fallback decoder to be sent cross-thread and
initialized without requiring extra thread hops back to report the
status of an intermediate failure.

Bug: 832917
Change-Id: Ib9e014a746e38588ce285153230511b33db8e47b
Reviewed-on: https://chromium-review.googlesource.com/1023115
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555948}