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This CL restores disabled TrayAccessibilityTest browser tests by using
SystemTrayTestApi mojo interface.

Although now the style of the test is Mash compliant, it still doesn't
work with Mash because some a11y features are not yet ported to Mash.

TEST=browser_tests
BUG=850014

Change-Id: I25bc3ec4b7409eb97632e9b14fc91aedf8fed178
Reviewed-on: https://chromium-review.googlesource.com/c/1309554Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605227}