[bluetooth] Boost priority while probing bthprops.cpl

The HasBluetoothStack() method checks if bthprops.cpl can be loaded.
This can take the dynamic library loader lock while on a background
thread and so an annotation is needed to temporarily boost thread
priority to avoid janking the main thread.

Noticed in crash/e8feafd49cc404d4.

Bug: 973868
Change-Id: I036ffacd5f22afa9a5493d746b2b1ada600ecfc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209133
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Vincent Scheib <scheib@chromium.org>
Reviewed-by: Vincent Scheib <scheib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770323}

device/bluetooth/bluetooth_init_win.cc

@@ -4,7 +4,7 @@
 
 #include "device/bluetooth/bluetooth_init_win.h"
 
-#include "base/threading/scoped_blocking_call.h"
+#include "base/threading/scoped_thread_priority.h"
 
 namespace {
 
@@ -28,8 +28,10 @@ bool HasBluetoothStack() {
   } has_bluetooth_stack = HBS_UNKNOWN;
 
   if (has_bluetooth_stack == HBS_UNKNOWN) {
-    base::ScopedBlockingCall scoped_blocking_call(
-        FROM_HERE, base::BlockingType::MAY_BLOCK);
+    // Mitigate the issues caused by loading DLLs on a background thread
+    // (http://crbug/973868).
+    SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY_REPEATEDLY();
+
     HRESULT hr = E_FAIL;
     __try {
       hr = __HrLoadAllImportsForDll("bthprops.cpl");