Add EME contentType checks for mp2t

We don't have 'mp2t/audio' for audio streams. For video streams, stream
parser supports encrypted AVC stream. The patch adds the supporting for
mp2t avc.

BUG=708261
TEST=Test on chromecast and Android.

Review-Url: https://codereview.chromium.org/2881443002
Cr-Commit-Position: refs/heads/master@{#473633}
2 files changed