Adding mojo MediaSessionClient to support media controls

third_party/WebKit/public/platform/modules/mediasession/media_session.mojom

Index: third_party/WebKit/public/platform/modules/mediasession/media_session.mojom
diff --git a/third_party/WebKit/public/platform/modules/mediasession/media_session.mojom b/third_party/WebKit/public/platform/modules/mediasession/media_session.mojom
index dae65f6e339dbc465103e023d64599ffba08892a..d416c4fb026619d81e4be4bbc150aa42d8b5f276 100644
--- a/third_party/WebKit/public/platform/modules/mediasession/media_session.mojom
+++ b/third_party/WebKit/public/platform/modules/mediasession/media_session.mojom
@@ -25,6 +25,16 @@ struct MediaMetadata {
   array<MediaImage> artwork;
 };
 
+interface MediaSessionClient {
+  DidReceivedAction(string action);

[whywhat, 2016/10/17 15:38:43]

Could you change action from string to enum?

[Zhiqiang Zhang (Slow), 2016/10/17 21:17:39]

Done. However it seems mapping values enum (or int) from/to AtomicString is ugly
in Blink. Also the tests are bit more unclear since it's using integer as enums.

See the new patch. I'd prefer to revert the change and do the mapping in
content/ in a later patch, WDYT?

+};
+
 interface MediaSessionService {
+  // MediaSessionClient interface is to used to notify Blink MediaSession of
+  // media control actions.
+  SetClient(MediaSessionClient client);
+
   SetMetadata(MediaMetadata? metadata);
+
+  DidSetEventHandlerForAction(string action, bool is_set);

[whywhat, 2016/10/17 15:38:43]

Could you change is_set to two methods?

Maybe the method should not be about the event handler at all but rather
something like:

EnableDisableAction(Action action, bool enable) // again, consider having Enable
and Disable methods

[Zhiqiang Zhang (Slow), 2016/10/17 21:17:39]

Done.

 };