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This pushes more functionality out of //chrome and down into
//ui/views. It will make it easier to share the code with
//ash, and possibly also for chromecast.

There are AXTreeSourceAura unit tests in //chrome that exercise
the integration of AXTreeSource with AXRootObjWrapper. I left
those in place but converted to ChromeViewsTestBase. This
eliminates a chrome-to-ash dependency. The tests can be moved
into //ui/views in a follow-up CL. I didn't do it now so the
diffs would be easier to read.

BUG=910672
TEST=unit_tests, views_unittests

Change-Id: Ib1483bc6b8e792409b106e0cdd12bff415032ef6
Reviewed-on: https://chromium-review.googlesource.com/c/1357499Reviewed-by: Sergey Volk <servolk@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612870}