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Move destroying input and output buffers from child thread to
decoder thread has two benefits. (1) Book accounting variables
like output_buffer_map_ are only accessed in decoder thread.
(2) kChangingResolution used to mean waiting for output buffers
to destroy or waiting for processor to return frames. Now it
only means waiting for processor. The code is simpler.

BUG=b/29059119
TEST=Run VDA unittest, run video_VideoSeek test and play video on
elm and peach pit.

Review-Url: https://codereview.chromium.org/2335573002
Cr-Commit-Position: refs/heads/master@{#420027}