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The Chai dependency prohibits the util from being used in WebUI tests
that are not configured to use Chai, like the PDF Viewer's.

Change the assertion to be part of a condition. The assertion was
already strong and unlikely to fail.

Change-Id: I008a6b6ccb1cd8c83d3ec2dcea3476243e89cb0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495506
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821049}