[media] Boost priority while calling CoCreateInstance

Calling CoCreateInstance() in EnumerateDirectShowDevices() can cause
DLLs to be loaded. Since this method is called on a background thread
this can cause a priority inversion of the main thread is also trying to
acquire the dynamic loader lock.

This annotation temporarily boosts the priority of the thread to allow
it to complete loading the library and drop the lock.

Noticed in crash/8c42f60e46efa272.

Bug: 973868
Change-Id: I4eef876860d712e639bcf404553b8d8d06b5864c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208649
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770471}

media/capture/video/win/video_capture_device_factory_win.cc

@@ -240,6 +240,10 @@ std::string GetDeviceModelId(const std::string& device_id) {
 }
 
 HRESULT EnumerateDirectShowDevices(IEnumMoniker** enum_moniker) {
+  // Mitigate the issues caused by loading DLLs on a background thread
+  // (http://crbug/973868).
+  SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY();
+
   ComPtr<ICreateDevEnum> dev_enum;
   HRESULT hr = ::CoCreateInstance(CLSID_SystemDeviceEnum, NULL, CLSCTX_INPROC,
                                   IID_PPV_ARGS(&dev_enum));