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This is a reland of f70764f0

Original change's description:
> Adds a class that encapsulates a range in Blink's accessibility tree.
> 
> This is just a basic class that can convert from a DOM selection to an AX range and vice versa. It can also perform a selection on the AX range.
> This class doesn't still handle any of the many corner cases that I will be looking at next, cases that will be covered by tests.
> R=dmazzoni@chromium.org
> 
> Bug: 
> Change-Id: I7c98617e8ef3b8bcca8cfc0d2e5518f2da771063
> Reviewed-on: https://chromium-review.googlesource.com/678016
> Commit-Queue: Nektarios Paisios <nektar@chromium.org>
> Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
> Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Nektarios Paisios <nektar@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#542619}

Change-Id: I8c6badcc06ad4b012ac73d69ca9e64c921353601
Reviewed-on: https://chromium-review.googlesource.com/959431Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542953}