Don't reset speech_recognition_manager_ in BrowserMainLoop::ShutdownThreadsAndCleanUp()

Instead, let it be reset during the destruction of BrowserMainLoop, which
happens after the IO thread has been shutdown.

This prevents a race condition between ShutdownThreadsAndCleanUp() and the
destructor of InputTagSpeechDispatcherHost. The latter runs on the IO thread,
and could sometimes end up trying to use the SpeechRecognitionManager
at the same time as it was being destructed.

Tested by running WorkerTest.QueuedSharedWorkerShutdown a couple of hundred
times under ASan, as well as running existing speech tests.

BUG=none


Review URL: https://chromiumcodereview.appspot.com/10836007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149394 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/browser_main_loop.cc

@@ -498,8 +498,6 @@ void BrowserMainLoop::ShutdownThreadsAndCleanUp() {
   if (resource_dispatcher_host_.get())
     resource_dispatcher_host_.get()->Shutdown();
 
-  speech_recognition_manager_.reset();
-
 #if defined(USE_AURA)
   ImageTransportFactory::Terminate();
 #endif