Fix layout test for Event.scoped

Fix the layout test related to this CL : https://codereview.chromium.org/1586563005/
Changes are made so that the test will handle the cases where events do not happen.
BUG=531990, 575042

Review URL: https://codereview.chromium.org/1641573002

Cr-Commit-Position: refs/heads/master@{#372627}
1 file changed