Migrate get_selection from BrowserAccessiblity to AXPlatformNode

Bug: 703369
Change-Id: Ie8609129a043c461e0da0d7f01462637642636fa
Reviewed-on: https://chromium-review.googlesource.com/659218
Commit-Queue: Doug Turner <dougt@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502243}
3 files changed