The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4bb33820986292a9fd983b2ca758dcee1cd196f7

commit 4bb33820986292a9fd983b2ca758dcee1cd196f7
Author: Katie D <katie@chromium.org>
Date: Fri Feb 09 19:30:18 2018

Immediately update focus & highlight rather than waiting for hittest.

Because we only set word highlighting after doing an automation
hittest, highlight results with faster speech rates could seem
inconsistent due to the hittest waiting to return.

Also fixes a nit with highlight, and a bug where STS could get stuck
thinking that the selection key was down. Does the latter by making
sure that any keyUp from the selection key updates selection key
state, whether or not selection should be read.

Bug:  810075 , 809732 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I45e62fc15d558ee548e60b4b4aea27ca7d65158d
Reviewed-on: https://chromium-review.googlesource.com/907337
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535783}
[modify] https://crrev.com/4bb33820986292a9fd983b2ca758dcee1cd196f7/chrome/browser/resources/chromeos/select_to_speak/select_to_speak.js
[modify] https://crrev.com/4bb33820986292a9fd983b2ca758dcee1cd196f7/chrome/browser/resources/chromeos/select_to_speak/select_to_speak_mouse_selection_test.extjs
[modify] https://crrev.com/4bb33820986292a9fd983b2ca758dcee1cd196f7/chrome/browser/resources/chromeos/select_to_speak/select_to_speak_unittest.gtestjs

Google Chrome 67.0.3369.0 (Official Build) canary (64-bit)
Firmware Version Google_Samus.6300.276.0
Flag enabled: #enable-experimental-accessibility-features

Able to quickly go back and forth with both methods to invoke STS: highlight text, press ctrl + s to start reading. While still reading, hold down search + highlight. STS stops reading as soon as search is pressed (as expected per 819925). It is then able to start reading the mouse highlighted text. I was able to go back and forth in this way 4 times in a row. 

Moving from just having STS label to also having the UI>Accessibility>SelectToSpeak component to make searching easier in the future.