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Previously, only some accelerators worked in Windows/Linux [e.g. ctr + V works,
ctr + T did not]. All accelerators were busted on macOS Views. Some accelerators
were busted on macOS Cocoa.

The root cause is that SigninViewControllerDelegateViews::HandleKeyboardEvent
was not implemented on Views, and
SigninViewControllerDelegateMac::HandleKeyboardEvent was incorrectly implemented
on macOS.

This CL fixes both implementations to use the now standard logic for
redispatching key events. This CL also fixes the implementation of
CommandDispatcher on macOS to correctly detect whether an event is being
redispatched.

Change-Id: I76b1ef263fce171ae182d316f541f836ab6388ee
Bug: 776304
Reviewed-on: https://chromium-review.googlesource.com/1097206
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568052}