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This CL switches MediaResourceGetter to use TaskScheduler and BindOnce.

BUG=689520

Change-Id: Icbc98cb32de3a3e7b2a91e2692446747794f62eb
Reviewed-on: https://chromium-review.googlesource.com/559993Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484970}