GitLab

Replicates FFmpeg's behavior for frame allocation.  They do not zero
the frame on subsequent pool reuse, so I have not done so either.

BUG=390941,390944,390945
TEST=new unittest.

Review URL: https://codereview.chromium.org/383893002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282775 0039d316-1c4b-4281-b951-d872f2087c98