Improved performance of AXPosition::operator less than

Only move up or down the tree using text equivalent positions (by calling CreateParentPosition and AsLeafTextPosition) when we absolutely need to.
R=dmazzoni@chromium.org

Tested: VoiceOver on Mac by navigating in Gmail and checking how many times you hear "Busy"
Change-Id: Id9469e52ef87b8b8a0c6c5fac5325abec415382d
Bug: 861567
Reviewed-on: https://chromium-review.googlesource.com/1133334
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574689}
2 files changed