Add 'Slow' to a test that timeouts after Skia roll.
Add 'Slow' to a test that timeouts after Skia roll.

BUG=259094
TBR=robertfillips@chromium.org

Review URL: https://codereview.chromium.org/18945003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@153936 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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