Clear MediaStreamManager's request list during IO-thread shutdown.

Previously there was a DCHECK expecting the request list to be empty,
which is causing some tests to flake.

Bug: 778869
Change-Id: I53276aded82752a5a7f01a58e610950e59d062db
Reviewed-on: https://chromium-review.googlesource.com/746663Reviewed-by: Tommi <tommi@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512843}

content/browser/renderer_host/media/media_stream_manager.cc

@@ -1584,7 +1584,6 @@ void MediaStreamManager::StopMediaStreamFromBrowser(const std::string& label) {
 void MediaStreamManager::WillDestroyCurrentMessageLoop() {
   DVLOG(3) << "MediaStreamManager::WillDestroyCurrentMessageLoop()";
   DCHECK(CalledOnIOThread());
-  DCHECK(requests_.empty());
   if (media_devices_manager_)
     media_devices_manager_->StopMonitoring();
   if (video_capture_manager_)
@@ -1596,6 +1595,7 @@ void MediaStreamManager::WillDestroyCurrentMessageLoop() {
   video_capture_manager_ = nullptr;
   media_devices_manager_ = nullptr;
   g_media_stream_manager_tls_ptr.Pointer()->Set(nullptr);
+  requests_.clear();
 }
 
 void MediaStreamManager::NotifyDevicesChanged(