Migrate get_nCharacters from BrowserAccessiblity to AXPlatformNode

Bug: 703369
Change-Id: I87321e56b172a0fd8f52903d3e89fc7540391d30
Reviewed-on: https://chromium-review.googlesource.com/653726
Commit-Queue: Doug Turner <dougt@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#500777}
3 files changed