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When we allocate frames with new layout, we always need to know the
maximum number of frames. This CL combines SetMaxNumFrames() and
NegotiateFrameFormat() to RequestFrames() to simplify the interface.

BUG=b:136716638
TEST=media_unittests --gtest_filter=PlatformVideoFramePoolTest*
TEST=video_decode_accelerator_tests on Kevin and Eve

Change-Id: I7c194a9107319f1a91da961140548216d2240e8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1861437
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711656}