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Currently we always set a "just-enough" frame number. In usual usage
there is no stale frames. This CL remove the frame purging logic to
simplify the code and reduce the periodically cleanup time.

BUG=none
TEST=media_unittests --gtest_filter=PlatformVideoFramePoolTest
TEST=Run video_decode_accelerator_tests on Kevin

Change-Id: I1542c97ed2a492776bc6ec7a2e1d48fdc12467a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786706
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: David Staessens <dstaessens@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695060}