Propagate cast application stop

Currently, if a cast application is stopped from a different device that
started the application, RemotePlayback is not properly torn down. This
is because the cast application listener is never given a reference to
the session that it should tear down.

This CL fixes the issue, and adds a pre-emptive null check.

Bug: 790766
Change-Id: I6fa67ca87a05efd135c57b396d6d09e69065a6f3
Reviewed-on: https://chromium-review.googlesource.com/c/1327605Reviewed-by: Zhiqiang Zhang <zqzhang@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607035}

chrome/android/java/src/org/chromium/chrome/browser/media/router/cast/ChromeCastSessionManager.java

@@ -146,6 +146,7 @@ public class ChromeCastSessionManager {
         mSession = session;
         mSessionLaunching = false;
         mCurrentSessionListener.onSessionStarted(session);
+        mListener.setSession(session);
     }
 
     /**
@@ -173,7 +174,7 @@ public class ChromeCastSessionManager {
      * Stops the current session.
      */
     public void stopApplication() {
-        mSession.stopApplication();
+        if (mSession != null) mSession.stopApplication();
     }
 
     /**