This change refactors the Mac command dispatcher delegate to allow application
shortcuts to override builtin shortcuts. This always worked for non-"window"
shortcuts, but window shortcuts were dispatched in prePerformKeyEquivalent:,
so they had higher priority than menu key equivalents. This change causes
prePerformKeyEquivalent: to check for a menu key equivalent and execute that
before consulting the window shortcut table. Since none of Chrome's default key
equivalents overlap the window shortcuts, any menu item whose key equivalent
matches a window shortcut is user-added and should have absolute priority.

Bug: 410357
Change-Id: I853168f1aee6c2fa82757f3fd41e04d7fc2a79be
Reviewed-on: https://chromium-review.googlesource.com/716736


Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508801}