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This makes it so that if an app requests the WindowService it's
launched in Ash. This is only enabled if --mash and --mus is not
specified.

BUG=837686
TEST=none

Change-Id: Ic01879e0aff8828b17c0652bd74078daba628d40
Reviewed-on: https://chromium-review.googlesource.com/1045248
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556338}