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This adds a queue to the write and read sides of
MojoDecoderBufferConverter, allowing for multiple concurrent buffers to
be processed.

Writes are no longer guaranteed to fail as soon as the pipe is closed,
since no write is attempted immediately. Since there is no error
reporting mechanism on the write side, the only guarantee is that
*eventually* writes will fail. Since MojoDecoderBufferConverter is not
a primary communication channel, this isn't likely to cause
complications.

Bug: 751926
Change-Id: I11c985d8aba3acc8fba6453f5bcf22555334b5b0
Reviewed-on: https://chromium-review.googlesource.com/679735
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505420}