cros: Lower the priority for wallpaper controller task runner

Bug: 910409
Change-Id: I7c9a65ccb5e6eb11e2e3f600cef3ba13fe37ee82
Reviewed-on: https://chromium-review.googlesource.com/c/1356240Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612692}

ash/wallpaper/wallpaper_controller.cc

@@ -476,7 +476,7 @@ WallpaperController::WallpaperController()
       color_profiles_(GetProminentColorProfiles()),
       wallpaper_reload_delay_(kWallpaperReloadDelay),
       sequenced_task_runner_(base::CreateSequencedTaskRunnerWithTraits(
-          {base::MayBlock(), base::TaskPriority::USER_BLOCKING,
+          {base::MayBlock(), base::TaskPriority::USER_VISIBLE,
            base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN})),
       scoped_session_observer_(this),
       weak_factory_(this) {