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This change:

1) Adds logic to EventMonitorMac so that if an event is marked handled by the
   client of EventMonitorMac, the event is not handled further - i.e.,
   EventMonitorMac can now eat events;
2) Renames the existing MenuPreTargetHandler to MenuPreTargetHandlerAura;
3) Adds MenuPreTargetHandlerMac which uses EventMonitorMac;
4) Adds MenuPreTargetHandler::Create to manufacture the platform-appropriate
   MenuPreTargetHandler instance.

Bug: 867127
Change-Id: I33f7c750598b0496fa291434098f8019e4b99c47
Reviewed-on: https://chromium-review.googlesource.com/1174568
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583254}