views: close menus when accelerators are pressed on Mac

This change:

1) Has MenuController consider command to be an accelerator key, along with
   control and alt, for the purpose of not searching for mnemonics;
2) Adds a Mac override of ViewsDelegate::ProcessAcceleratorWhileMenuShowing
   which tells the menu to close for an accelerator

Bug: 885138
Change-Id: Iee9d9abb3b1bb3c2d346d7aaf7e532866b4ad26a
Reviewed-on: https://chromium-review.googlesource.com/c/1334147
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607699}
3 files changed