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This allows testing ChromeVox with multiple simultaneous mash apps.

* Turn on a11y support in AuraInit::InitParams
* Initialize ash window property transport

Bug: 888147, 892743
Change-Id: I3114787aa687dd6a233b7411739fa3f938cf4399
Reviewed-on: https://chromium-review.googlesource.com/c/1265535
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597341}