Since ffmpeg use an optimized bitstream reader that reads in chunks of 32/64
bits the bitstream buffer has to be increased in order for the reader to not
read out of bounds.
BUG=webrtc:5514
R=stefan@webrtc.org
Review URL: https://codereview.webrtc.org/2476513004 .
Cr-Commit-Position: refs/heads/master@{#14941}