Add HW fallback option to software decoding.

Permits falling back to software decoding for unsupported resolutions in
bitstreams.

BUG=4625, chromium:487934
R=mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46269004

Cr-Commit-Position: refs/heads/master@{#9209}
10 files changed