Create a null decoder for h264 when not supported, instead of crashing.

BUG=
NOTRY=true

Review-Url: https://codereview.webrtc.org/2091923002
Cr-Original-Commit-Position: refs/heads/master@{#13311}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0dacbf5f62644c4770496d883721666d98d8a63e
1 file changed