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WM_QUIT messages are intended to be used internally by applications to
exit message-pumping loops built using the GetMessage() API.

Chromium MessageLoop/RunLoop have a common cross-platform Quit()
mechanism, and much of our code assumes that loops will run until we
explicitly ask them to Quit(), so we have no need to process WM_QUIT.

We might see WM_QUIT messages injected by other applications, for some
reason, and some code we run will use a GetMessage() loop and exit that
via PostQuitMessage(). If a nested GetMessage() loop triggers multiple
PostQuitMessage() calls before returning then we may end up with our
MessagePumpForUI receiving WM_QUIT unintentionally.

Bug: 720078
Change-Id: I11df406ab4fd1d0a3b56899bf1a710ef51c048e9
Reviewed-on: https://chromium-review.googlesource.com/1020452
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553653}