Explicitly specify priority and shutdown behavior for SystemHotkeyHelperMac...

Explicitly specify priority and shutdown behavior for SystemHotkeyHelperMac PostDelayedTaskWithTraits.

This is a follow up CL to the comments in https://chromium-review.googlesource.com/c/564177 after it was committed.

Bug: 
Change-Id: I8f8b41dfd1fe70a4bf65ed3b228bb14aca0243ec
Reviewed-on: https://chromium-review.googlesource.com/567020Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486033}

content/browser/cocoa/system_hotkey_helper_mac.mm

@@ -34,7 +34,9 @@ SystemHotkeyHelperMac* SystemHotkeyHelperMac::GetInstance() {
 
 void SystemHotkeyHelperMac::DeferredLoadSystemHotkeys() {
   base::PostDelayedTaskWithTraits(
-      FROM_HERE, {base::MayBlock()},
+      FROM_HERE,
+      {base::TaskPriority::USER_VISIBLE,
+       base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN, base::MayBlock()},
       base::Bind(&SystemHotkeyHelperMac::LoadSystemHotkeys,
                  base::Unretained(this)),
       base::TimeDelta::FromSeconds(kLoadHotkeysDelaySeconds));