Explicitly signal HE-AAC in DASH manifest

This works around an issue in the Chromium MediaSource implementation[1].
Chromium will not play a video when the audio track uses HE-AAC, unless it is
explicitly signaled in the manifest.

[1]: https://code.google.com/p/chromium/issues/detail?id=370927#c3
Merge pull request #458 from saintdev/for-upstream

Explicitly signal HE-AAC in DASH manifest