Ensures BluetoothAdapterWinrt initialisation runs on foreground thread

PerformSlowInitTasks should be scheduled (BEST_EFFORT) but once it
starts executing it needs to be on a foreground thread to avoid
priority inversion.

From this crash/312ca3b6fb0c2131
  Thread 28 ThreadPoolBackgroundWorker

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

This is blocking other threads.

R=joenotcharles@google.com

Bug: 973868
Change-Id: Ic7512ad567bc4e3a5fc549dd9c408db0047a45fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377904Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802019}

device/bluetooth/bluetooth_adapter_winrt.cc

@@ -31,7 +31,6 @@
 #include "base/task/post_task.h"
 #include "base/task/task_traits.h"
 #include "base/task/thread_pool.h"
-#include "base/threading/scoped_thread_priority.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/win/core_winrt_util.h"
 #include "base/win/post_async_results.h"
@@ -660,7 +659,9 @@ void BluetoothAdapterWinrt::Initialize(base::OnceClosure init_callback) {
   // Some of the initialization work requires loading libraries and should not
   // be run on the browser main thread.
   base::ThreadPool::PostTaskAndReplyWithResult(
-      FROM_HERE, {base::MayBlock(), base::TaskPriority::BEST_EFFORT},
+      FROM_HERE,
+      {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
+       base::ThreadPolicy::MUST_USE_FOREGROUND},
       base::BindOnce(&BluetoothAdapterWinrt::PerformSlowInitTasks),
       base::BindOnce(&BluetoothAdapterWinrt::CompleteInitAgile,
                      weak_ptr_factory_.GetWeakPtr(), std::move(init_callback)));
@@ -700,10 +701,6 @@ void BluetoothAdapterWinrt::InitForTests(
 // static
 BluetoothAdapterWinrt::StaticsInterfaces
 BluetoothAdapterWinrt::PerformSlowInitTasks() {
-  // Mitigate the issues caused by loading DLLs on a background thread
-  // (http://crbug/973868).
-  SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY();
-
   if (!ResolveCoreWinRT())
     return BluetoothAdapterWinrt::StaticsInterfaces();