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The newly shareable attribute code can be used for ATK on Aura Linux
because the attributes exposed should be exactly the same on this
platform. This change also adds some basic unit tests to ensure that
attributes are exposed properly through ATK.

Bug: 866612
Change-Id: Id46ab2108d9ead6ab88b5e172b6b25e615f47cca
Reviewed-on: https://chromium-review.googlesource.com/1156399
Commit-Queue: Martin Robinson <mrobinson@igalia.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580164}