Leak UserInputMonitor on AudioManager thread hangs

There may be live streams still using it, causing crashes. As far as I
can see, this is the only dependency of audio streams which might be
used this way.

BUG=745676
R=dalecurtis

Change-Id: I70dfe49331de863a707cf6c2945e06c83b87c5b4
Reviewed-on: https://chromium-review.googlesource.com/579198Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488715}

content/browser/browser_main_loop.cc

@@ -1373,6 +1373,9 @@ void BrowserMainLoop::ShutdownThreadsAndCleanUp() {
       // Intentionally leak AudioManager if shutdown failed.
       // We might run into various CHECK(s) in AudioManager destructor.
       ignore_result(audio_manager_.release());
+      // |user_input_monitor_| may be in use by stray streams in case
+      // AudioManager shutdown failed.
+      ignore_result(user_input_monitor_.release());
     }
   }