Disable caching of video-device enumerations on Mac.

There is an issue with the Mac video device monitor that is causing
cache inconsistencies.

Bug: 834581
Change-Id: Idc1a0d953509fee7787fb07d19376d4800f76939
Reviewed-on: https://chromium-review.googlesource.com/1023855Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552689}

content/browser/renderer_host/media/media_devices_manager.cc

@@ -396,6 +396,9 @@ void MediaDevicesManager::StartMonitoring() {
   }
 
 #if defined(OS_MACOSX)
+  // TODO(crbug.com/835765): Do not force NO_CACHE for video input once the
+  // monitor for video-input devices is fixed.
+  SetCachePolicy(MEDIA_DEVICE_TYPE_VIDEO_INPUT, CachePolicy::NO_CACHE);
   BrowserThread::PostTask(
       BrowserThread::UI, FROM_HERE,
       base::Bind(&MediaDevicesManager::StartMonitoringOnUIThread,