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This is a big "many-cases-in-one" test that takes several seconds to
run. It appears to occasionally timeout when run as part of web tests.
This adds the test to SlowTests.

Bug: 1133836
Change-Id: I5a6408ac69228f50e35eb6ec63bd84bc95554dd2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443542Reviewed-by: Lan Wei <lanwei@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813062}