Revert of Remove secondary player from Media Sessions

Revert of Remove secondary player from Media Sessions (patchset #1 id:1 of https://codereview.chromium.org/1836093003/ )

Reason for revert:
Didn't build on M50.

Original issue's description:
> Remove secondary player from Media Sessions
> 
> When we switch Android media players, for cast, we no longer want audio
> focus changes associated with old player to take effect. To handle this
> remove the old player from the Media Sessions.
> 
> The new player is responsible for adding itself to the Media Sessions if
> it needs to. The remote player doesn't need to, and, after stopping casting
> the local player will do so when the user presses play.
> 
> BUG=595373
> 
> Review URL: https://codereview.chromium.org/1823983002
> 
> Cr-Commit-Position: refs/heads/master@{#382613}
> (cherry picked from commit 4bee90cc8b21594a3fc5c4055887886aa2009865)
> 
> Committed: https://chromium.googlesource.com/chromium/src/+/3ad63a558b2fbe889d02eeffe2f664955bfc9f07

TBR=boliu@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=595373

[aberent, 2016-03-29 17:45:54]

Created Revert of Remove secondary player from Media Sessions

[commit-bot: I haz the power, 2016-03-29 17:46:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1842763003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1842763003/1

[commit-bot: I haz the power, 2016-03-29 17:47:39]

Committed patchset #1 (id:1)