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

content/public/android/java/src/org/chromium/content/browser/MediaSessionObserver.java

+// 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.
+
+package org.chromium.content.browser;
+
+import org.chromium.content_public.common.MediaMetadata;
+
+/**
+ * Java MediaSessionObserver. The observer is proxied via Java ChromeMediaSession and does not have
+ * a native counterpart.
+ */
+public class MediaSessionObserver {
+    // The observed MediaSession.
+    private ChromeMediaSession mSession;
+
+    /**
+     * Create a MediaSessionObserver observing |session|.
+     */
+    public MediaSessionObserver(ChromeMediaSession session) {
+        session.addObserver(this);
+        mSession = session;
+    }
+
+    /**
+     * Get the observed MediaSession.
+     */
+    public final ChromeMediaSession getSession() {
+        return mSession;
+    }
+
+    /**
+     * Called when MediaSession is destroyed.
+     */
+    public void mediaSessionDestroyed() {}
+
+    /**
+     * Called when the native {@link MediaSession} state has changed.
+     * @param isControllable Whether the native {@link MediaSession} is controllable.
+     * @param isSuspended Whether the native {@link MediaSession} is suspended.
+     */
+    public void mediaSessionStateChanged(boolean isControllable, boolean isSuspended) {}
+
+    /**
+     * Called when the native {@link MediaSession} has changed metadata.
+     * @param metadata The new metadata of the native {@link MediaSession}. "null" is for unsetting
+     * metadata.
+     */
+    public void mediaSessionMetadataChanged(MediaMetadata metadata) {}
+}