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Introduces SelectionDeserializer that receives an HTML snippet and creates a selection in the AX tree

Will help with automated testing using HTML files that include selection markers.
R=dmazzoni@chromium.org

Change-Id: I7dd23b980b6966c87e09c12bdb27b8ad27c9cc15
Reviewed-on: https://chromium-review.googlesource.com/1106901
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575194}