Make Views menus close when any Mac menu opens.

Before, Views menus closed when the main menu was opened; make it
so that they close when any menu opens.

BUG=985675
TEST=as in bug

Change-Id: I269a1e699def76abc79c4e496ec734936ae18f59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775735
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692058}

ui/views/controls/menu/menu_cocoa_watcher_mac.mm

@@ -15,7 +15,7 @@ MenuCocoaWatcherMac::MenuCocoaWatcherMac(base::OnceClosure callback)
     : callback_(std::move(callback)) {
   observer_token_other_menu_ = [[NSNotificationCenter defaultCenter]
       addObserverForName:NSMenuDidBeginTrackingNotification
-                  object:[NSApp mainMenu]
+                  object:nil
                    queue:nil
               usingBlock:^(NSNotification* notification) {
                 ExecuteCallback();