Move audio focus control from media/ to content/ and make it per WebContents.

media/base/android/media_source_player.cc

Index: media/base/android/media_source_player.cc
diff --git a/media/base/android/media_source_player.cc b/media/base/android/media_source_player.cc
index 7e8f49a4613c4a6c3189ea49ec4c94024688a249..460fbec8fb6ced9c158d034b403ee382df4c37e7 100644
--- a/media/base/android/media_source_player.cc
+++ b/media/base/android/media_source_player.cc
@@ -217,6 +217,11 @@ void MediaSourcePlayer::StartInternal() {
   if (pending_event_ != NO_EVENT_PENDING)
     return;
 
+  if (!manager()->RequestPlay(player_id())) {
+    Pause(true);

[qinmin, 2015/05/23 18:16:06]

i think this should be false, the value is use by remote_media_player_bridge.
If the local player loses AudioFocus, it shouldn't impact the remote player. 

[mlamouri (slow - plz ping), 2015/05/23 20:05:27]

This question might sound silly but what's the link between this and
RemoteMediaPlayerBridge? I wasn't able to find why passing true or false would
have any impact there.

+    return;
+  }
+
   // When we start, we could have new demuxed data coming in. This new data
   // could be clear (not encrypted) or encrypted with different keys. So key
   // related info should all be cleared.