[ui] Boost priority on the first call to BuildDeviceToPathMapOnBackgroundThread

DLL loading on background thread causing priority inversions that jank
the main thread, see https://crbug.com/973868#c31 for a snapshot of
a jank caused by this one.

Bug: 973868
Change-Id: Id4879953754fb6bef09ef95fe998befce508e047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2152904Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760046}

ui/display/win/color_profile_reader.cc

@@ -11,6 +11,7 @@
 #include "base/files/file_util.h"
 #include "base/task/post_task.h"
 #include "base/task/thread_pool.h"
+#include "base/threading/scoped_thread_priority.h"
 #include "ui/display/win/display_info.h"
 #include "ui/gfx/icc_profile.h"
 
@@ -79,6 +80,7 @@ void ColorProfileReader::UpdateIfNeeded() {
 // static
 ColorProfileReader::DeviceToPathMap
 ColorProfileReader::BuildDeviceToPathMapOnBackgroundThread() {
+  SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY();
   DeviceToPathMap device_to_path_map;
   EnumDisplayMonitors(nullptr, nullptr, EnumMonitorForProfilePathCallback,
                       reinterpret_cast<LPARAM>(&device_to_path_map));