Add accessibility_perftests

Recently there have been performance improvements in ui/accessibility.
This CL creates BUILD.gn entry for accessibility_perftests.exe which
will be used to run performance tests for this directory.

With this CL, I'm adding perf tests for the following methods :
AXPosition<>::AsTreePosition
AXPosition<>::AsLeafTextPosition

Bug: 928948
Change-Id: Iedbb274868a42009766de31d6f8b8731640847e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838264
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Adam Ettenberger <adettenb@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#708232}

• BUILD.gn[diff]
• ui/accessibility/BUILD.gn[diff]
• ui/accessibility/ax_node_position_perftest.cc[Added - diff]