FlameChart:[REGRESSION] OverviewPane was empty after r161616

BUG=320951
R=alph@chromium.org

Review URL: https://codereview.chromium.org/77013002

git-svn-id: svn://svn.chromium.org/blink/trunk@162404 bbb929c8-8fbe-4397-9dbb-9b2b20218538