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This makes test code easier to write and understand and is also what the
"prod" WebUI class already does.

This is loosely related to Dark Mode support, as I discovered this
difference in behavior while working on unit tests for this work.

WIP Dark Mode CL: https://crrev.com/c/1354685

R=jam@chromium.org
BUG=883049

Change-Id: I6862e2cd0244bdb02bd6dd92870bc2ec70d2099a
Reviewed-on: https://chromium-review.googlesource.com/c/1359132Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613332}