This CL allows Audio and VideoRenderer to be reinitialized in the
flushed/stopped states and allows switching between different audio
and video tracks during playback (but only a single track of each type
may be active at a time).

BUG=487288

Review-Url: https://codereview.chromium.org/2684103005
Cr-Commit-Position: refs/heads/master@{#463658}