Migrate get_caretOffset from BrowserAccessiblity to AXPlatformNode

Bug: 703369
Change-Id: If672c2f92fa5f74db6e89af266827e706d43c40e
Reviewed-on: https://chromium-review.googlesource.com/658042
Commit-Queue: Doug Turner <dougt@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503715}
3 files changed