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Also moves highlight related tests to a new file, and add test utils
files for shared code.

Bug: 978168
Tests: added new tests

Change-Id: Ie9e8bfea304334ca2210c08e5f5ffefa7d102e19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1689952
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683011}