This CL implements one-shot audio focus type inside MediaSession,
instead of letting players joining MediaSession using Persisitent
type, while ignoring audio focus changes.

In this CL, when a one-shot player joins MediaSession,
MediaSession will takes audio focus and becomes active. However
when a MediaSession has one-shot players, it will become
uncontrollable.

BUG=596516,619084

Review-Url: https://codereview.chromium.org/2475473002
Cr-Commit-Position: refs/heads/master@{#431260}