Mac h264: Do not retain CVPixelBufferRefs

This (in particular, sending the retained CVPixelBufferRefs over to an
AVSampleBufferDisplayLayer) may be causing IOSurface leaks in M52.

BUG=631485

Review URL: https://codereview.chromium.org/2197893002 .

Cr-Commit-Position: refs/branch-heads/2743@{#714}
Cr-Branched-From: 2b3ae3b8090361f8af5a611712fc1a5ab2de53cb-refs/heads/master@{#394939}
1 file changed