Migrate get_nSelections from BrowserAccessibilityWin to AXPlatformNode

Bug: 703369
Change-Id: Ide9bb656b685be1d436f4dc3e6fd2b242c49d09e
Reviewed-on: https://chromium-review.googlesource.com/659217
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Doug Turner <dougt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503328}
3 files changed