Fixing MediaSessionDelegateAndroid referencing destroyed MediaSession

There's a crash caused by asynchronous audio focus events.
MediaSessionDelegate may receive audio focus change signals after
MediaSession::~MediaSession is called. This CL adds checks to see if the
native MediaSession is destroyed before calling it's methods.

BUG=624208

Review-Url: https://codereview.chromium.org/2111163002
Cr-Commit-Position: refs/heads/master@{#403180}
1 file changed