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--mash causes thousands of lines of UI code to behave differently, but
in the best case there's no visual indication of what mode you're in.

Add an indicator to the status area that shows if you're running --mash.
This could be generalized later to a "flag warning", like we use in
browser windows when running --no-sandbox.

Bug: 808158
Test: ash_unittests
Change-Id: I477158085bed9665c2fc9e7d2356c1f346083919
Reviewed-on: https://chromium-review.googlesource.com/905631
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534860}