Boost thread priority while loading COMBase

This CL is boosting the thread priority to avoid loading the DLL at
 background priority.

The following stackframe is the cause of the priority inversion:

  base::win::RoGetActivationFactory
  device::BluetoothAdapterWinrt::PerformSlowInitTasks()
   [...]
  base::internal::WorkerThread::RunWorker()
  base::internal::WorkerThread::RunBackgroundPooledWorker

see: https://bugs.chromium.org/p/chromium/issues/detail?id=973868#c42
Noticed in crash/bc6506f76abf6a60

R=joenotcharles@chromium.org,robliao@chromium.org

Bug: 973868
Change-Id: Ifbe2b87aed7364ad24a59ad300512ef82c03491a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377891Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802021}

base/win/core_winrt_util.cc

@@ -4,11 +4,18 @@
 
 #include "base/win/core_winrt_util.h"
 
+#include "base/threading/scoped_thread_priority.h"
+
 namespace {
 
 FARPROC LoadComBaseFunction(const char* function_name) {
-  static HMODULE const handle =
-      ::LoadLibraryEx(L"combase.dll", nullptr, LOAD_LIBRARY_SEARCH_SYSTEM32);
+  static HMODULE const handle = []() {
+    // Mitigate the issues caused by loading DLLs on a background thread
+    // (http://crbug/973868).
+    SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY();
+    return ::LoadLibraryEx(L"combase.dll", nullptr,
+                           LOAD_LIBRARY_SEARCH_SYSTEM32);
+  }();
   return handle ? ::GetProcAddress(handle, function_name) : nullptr;
 }