TimelinePanel: followup ui fix for r166684

BUG=326121
TBR=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/135363009

git-svn-id: svn://svn.chromium.org/blink/trunk@166695 bbb929c8-8fbe-4397-9dbb-9b2b20218538