Update AudioStreamMonitor to handle updates to nonexistent streams

A crash has been reported where apparently it is possible for
a state update to arrive for a stream after the stream has been
removed.

This CL makes AudioStreamMonitor handle this case instead of DCHECKing.

Bug: 795501
Change-Id: Id43c3c4a846af5d8e7d1d387d36ed31e45724ad8
Reviewed-on: https://chromium-review.googlesource.com/832186Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524719}

content/browser/media/audio_stream_monitor.cc

@@ -149,7 +149,9 @@ void AudioStreamMonitor::UpdateStreamAudibleStateOnUIThread(const StreamID& sid,
                                                             bool is_audible) {
   DCHECK(thread_checker_.CalledOnValidThread());
   auto it = streams_.find(sid);
-  DCHECK(it != streams_.end());
+  if (it == streams_.end())
+    return;
+
   it->second = is_audible;
   UpdateStreams();
 }