Replicate FFmpeg's video frame allocation strategy.

      This should avoid accidental overreads and overwrites due to our
VideoFrame's not being as large as FFmpeg expects.

BUG=368980
TEST=new regression test

Review URL: https://codereview.chromium.org/270193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@268831 0039d316-1c4b-4281-b951-d872f2087c98