[Media>UI] Migrate media notification tests to JUnit tests

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

Index: content/public/android/java/src/org/chromium/content_public/browser/MediaSessionObserver.java
diff --git a/content/public/android/java/src/org/chromium/content_public/browser/MediaSessionObserver.java b/content/public/android/java/src/org/chromium/content_public/browser/MediaSessionObserver.java
index b6235e3ec13976be7f29a54d5454cbd7e41f8d19..83ecd407a7e7c281ede31c22b07a4921eab308c6 100644
--- a/content/public/android/java/src/org/chromium/content_public/browser/MediaSessionObserver.java
+++ b/content/public/android/java/src/org/chromium/content_public/browser/MediaSessionObserver.java
@@ -23,6 +23,13 @@ public abstract class MediaSessionObserver {
      * Construct a MediaSessionObserver and start observing |mediaSession|.
      */
     protected MediaSessionObserver(MediaSession mediaSession) {
+        // In tests, the MediaSession might be a mocked object which is not instance of
+        // MediaSessionImpl.
+        //
+        // TODO(mlamouri): move addObserver()/removeObserver() methods to the {@link MediaSession}
+        // class so that we don't need this type check?
+        if (!(mediaSession instanceof MediaSessionImpl)) return;
+
         mMediaSession = (MediaSessionImpl) mediaSession;
         mMediaSession.addObserver(this);
     }