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On Mac, they discovered long ago that quitting after holding Cmd+Q was a bad
idea since apps underneath Chrome would get the repeated Cmd+Q key event and
would also close [1].  Their solution was to delay quitting until the shortcut
is released.  To give users confirmation that Chrome is quitting, all browser
windows are faded out.

The same issue occurs on Linux and Windows (I've unintentionally closed many
background apps writing CL [2]), so this CL implements the Mac solution.

[1] http://dev.chromium.org/developers/design-documents/confirm-to-quit-experiment
[2] https://chromium.googlesource.com/chromium/src.git/+/6cbd505d3cb04e3e108e61a74f82421b66f0f655

BUG=243164
R=sky

Change-Id: Ia3de4acaf463cd5b202743d44f763c6d52fbe963
Reviewed-on: https://chromium-review.googlesource.com/1066924Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560336}