DevTools: fix flakiness of timeline-receive-response-event.html

BUG=407915
TBR=caseq

Review URL: https://codereview.chromium.org/516323004

git-svn-id: svn://svn.chromium.org/blink/trunk@181233 bbb929c8-8fbe-4397-9dbb-9b2b20218538