Position info (item n of m) incorrect if hidden focusable items in list

Bug: 836997
Change-Id: I971fa7076f72d51829b36af8e379260d48ca25ec
Reviewed-on: https://chromium-review.googlesource.com/c/1450235
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628890}
bool AXTree::HasObserver(AXTreeObserver* observer) {
  return observers_.HasObserver(observer);
}

bool AXTree::HasObserver(AXTreeObserver* observer) {
  return observers_.HasObserver(observer);
}
