Split MediaContentType and AudioFocusType

The existing MediaSession::Type does not work well for
Pepper&WebRTC. Spliting MediaContentType for MediaPlayer and
AudioFocusType for MediaSession to gain more control of audio focus.

Explainer for this CL:
https://docs.google.com/a/google.com/document/d/1W4uRH8Wvgau0_GtMda4EOSXRtxFuFLhb9W77sJagJpI/edit?usp=sharing

BUG=626351

Review-Url: https://codereview.chromium.org/1996043002
Cr-Commit-Position: refs/heads/master@{#414800}
35 files changed