Report changes to default/communications audio devices on Windows

With this change, it is safe to enable the SYSTEM_MONITOR caching
policy for MediaDevicesManager.

Bug: 788687
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I3b067a566e825c5a8219705bc75ee4b66c198787
Reviewed-on: https://chromium-review.googlesource.com/822935Reviewed-by: Max Morin <maxmorin@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523716}

content/public/common/content_features.cc

@@ -158,7 +158,7 @@ const base::Feature kLazyParseCSS{"LazyParseCSS",
 // cache is considered invalid on every enumeration request.
 const base::Feature kMediaDevicesSystemMonitorCache{
   "MediaDevicesSystemMonitorCaching",
-#if defined(OS_MACOSX)
+#if defined(OS_MACOSX) || defined(OS_WIN)
       base::FEATURE_ENABLED_BY_DEFAULT
 #else
       base::FEATURE_DISABLED_BY_DEFAULT

media/audio/win/audio_device_listener_win.cc

@@ -135,6 +135,10 @@ STDMETHODIMP AudioDeviceListenerWin::OnDefaultDeviceChanged(
     did_run_listener_cb = true;
   }
 
+  base::SystemMonitor* monitor = base::SystemMonitor::Get();
+  if (monitor)
+    monitor->ProcessDevicesChanged(base::SystemMonitor::DEVTYPE_AUDIO);
+
   DVLOG(1) << "OnDefaultDeviceChanged() "
            << "new_default_device: "
            << (new_default_device_id