Mark timeline-start-time.html as flaky

Flakiness dashboard is here:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=virtual/deferred/inspector/timeline/timeline-start-time.html

BUG=346119

Review URL: https://codereview.chromium.org/176543005

git-svn-id: svn://svn.chromium.org/blink/trunk@167649 bbb929c8-8fbe-4397-9dbb-9b2b20218538