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This will create a -diff.txt and -pretty-diff.html file for any test
that has an actual output. Absence of expected output is fine, the diff
will show the actual output as all new.

Bug: 1127360
Change-Id: I7f73741ff6d24571a20160239ee17ae69c6804e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2428826
Commit-Queue: Luke Z <lpz@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811086}