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The tests did not previously use an established test framework, making
tasks like executing certain code before or after every test and testing
asynchronous code more difficult. It should also be easier for
developers familiar with JavaScript testing in general and WebUI testing
in particular to read and write JavaScript tests for DOM Distiller and
Reader Mode.

Follow-up work includes replacing the custom assert methods with Chai,
as well as writing more tests to cover existing functionality.

Bug: 1027612
Change-Id: I39acc923a0424881d2694281d9aa0323c8f9a913
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992504Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Cr-Commit-Position: refs/heads/master@{#762551}