Fix one more layering violation caught by check-blink-deps

core/platform may not depend on core/ even for testing.

BUG=301947
R=abarth@chromium.org, abarth

Review URL: https://codereview.chromium.org/25284004

git-svn-id: svn://svn.chromium.org/blink/trunk@158544 bbb929c8-8fbe-4397-9dbb-9b2b20218538