Fix MediaDevices* tests dependency on BrowserThread globals after ~BrowserThreadImpl().

content/browser/renderer_host/media/media_stream_manager.cc

Index: content/browser/renderer_host/media/media_stream_manager.cc
diff --git a/content/browser/renderer_host/media/media_stream_manager.cc b/content/browser/renderer_host/media/media_stream_manager.cc
index 2eca6993e5b7921d350c5e8ddff1c13a6789691a..5cdb6233f3fe692852b212393224af1ba79ffd78 100644
--- a/content/browser/renderer_host/media/media_stream_manager.cc
+++ b/content/browser/renderer_host/media/media_stream_manager.cc
@@ -1757,4 +1757,10 @@ void MediaStreamManager::SetGenerateStreamCallbackForTesting(
   generate_stream_test_callback_ = test_callback;
 }
 
+#if defined(OS_WIN)
+void MediaStreamManager::FlushVideoCaptureThreadForTesting() {
+  video_capture_thread_.FlushForTesting();
+}
+#endif
+
 }  // namespace content