Link MediaSessionTabHelper with native MediaSession [CL is going to be split]

content/browser/media/session/media_session_observer.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "content/browser/media/session/media_session_observer.h"
+
+#include "content/browser/media/session/media_session.h"
+
+namespace content {
+
+MediaSessionObserver::MediaSessionObserver(MediaSession* session)
+    : session_(nullptr) {
+  Observe(session);

[whywhat, 2016/10/21 18:28:15]

I don't feel strongly about removing this, TBH. Just be aware that it may result
in extra code like this:
https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chr...

(MediaSinkObserver in C++ calls Observe in ctor which calls
CastMediaRouteProvider.startObservingMediaSinks which may cause onSinksReceived
if they're known already - on desktop the sinks are returned via Mojo/IPC so the
MediaSinkObserver derived class gets fully constructed; on Android we have to
post a task...).

Perhaps when Observe doesn't have any side effects, it's fine and RAII
convenient.

+}
+
+MediaSessionObserver::~MediaSessionObserver() {
+  if (session_)
+    session_->RemoveObserver(this);
+}
+
+void MediaSessionObserver::MediaSessionDisconnected() {
+  session_ = nullptr;
+}
+
+void MediaSessionObserver::Observe(MediaSession* session) {
+  session->AddObserver(this);
+  session_ = session;
+}
+
+}  // namespace content