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This change causes menu widgets to be become the active widget if the
menu was shown by keyboard. This ensures that the menu widget is
announced by screenreaders. There is one special case: editable
combobox menus, even if opened by keyboard, never activate since that
would pull keyboard focus away from the editable combobox itself.

To support that, this change also:
* Adds Widget::Predicate, a type for predicates that accept a Widget;
* Adds support to MenuCocoaWatcherMac for ignoring certain widget
  activations;
* Adds an IsNotMenuWidget predicate to MenuController for use with
  MenuCocoaWatcherMac

Bug: 1097769
Change-Id: I4f5289d66428f97e75c38e4e887b8ce9061dada8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520141
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824630}