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Set up the GTest and Mocha-based a11y testing framework for Chrome WebUI
by defining an accessibility_browsertest.js file for a11y test fixtures
and including this file in the BUILD.gn.

The passwords section now fires an event when the settings section has
expanded--an event that is responded to in the added a11y test.

The tests are disabled and will be enabled in followup CL for easier
review.

Bug: 737739
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ief6b002d8ce17950203c917a9baee05e3ac9136c
Reviewed-on: https://chromium-review.googlesource.com/598638
Commit-Queue: Tina Quach <quacht@google.com>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491539}