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This is part of moving ash out-of-process for mustash.

* Introduce FirstRunHelper mojo interface
* Make ash::FirstRunHelper object owned by ash::Shell
* Add explicit create/close widget methods because object is now
  persistent
* Add system tray test API to check if bubble is open

Next step is to move the widget creation into chrome.

Bug: 756057
Test: ash_unittests, browser_tests
Change-Id: Ie3994bd36b6dfeecd1c685773c5ce8e19394052e
Reviewed-on: https://chromium-review.googlesource.com/998181
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551091}