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This change exposes the AtkText interface on all accessibility elements
and uses the, now shared, ComputeHypertext in order to compute composite
text for elements that have multiple children. Once we start exposing
the hypertext interface, we can start testing the embedding of objects
into the hypertext itself. The plan is also to eventually expose the
interface on objects that need it.

Unfortunately, since hypertext computation is now triggered by the
content code, we must move the existing AtkText unit test to content as
well alongside the new one.

Bug: 866337
Change-Id: I8cc738dd0e14cf95ecdffa2370d242fb447aa9b6
Reviewed-on: https://chromium-review.googlesource.com/1190782
Commit-Queue: Martin Robinson <mrobinson@igalia.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586772}