macOS: Disable UI updates during popup-menu fade out.

There's a large memory leak on macOS of CATransaction continuations.
There appears to be correlation with PopupMenus, and it begins in
approximately the date range the CL that introduced this functionality
was added.

Bug: 853438
Change-Id: I7f9b63584e6990f1ee87b4d33a5b8405f81efdb9
Reviewed-on: https://chromium-review.googlesource.com/1117608Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571015}

content/browser/frame_host/popup_menu_helper_mac.mm

@@ -88,7 +88,10 @@ void PopupMenuHelper::ShowPopupMenu(
     base::mac::ScopedSendingEvent sending_event_scoper;
 
     // Ensure the UI can update while the menu is fading out.
-    pump_in_fade_ = std::make_unique<base::ScopedPumpMessagesInPrivateModes>();
+    // TODO(erikchen): Temporarily disabled as potential cause of memory leak of
+    // CATransaction continuations. See https://crbug.com/853438.
+    // pump_in_fade_ =
+    // std::make_unique<base::ScopedPumpMessagesInPrivateModes>();
 
     // Now run a NESTED EVENT LOOP until the pop-up is finished.
     [runner runMenuInView:cocoa_view