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This is a clean up CL so that we can avoid passing one additional
callback into MojoCdmFileIO.

BUG=721876
TEST=No functionality change

Change-Id: Ifa3b071a5a95437f87b50f888bce72bc51ad3fd9
Reviewed-on: https://chromium-review.googlesource.com/770318
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517324}