TimelineFlameChart: draw frame dividers in FlameChart

BUG=316167
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/177473002

git-svn-id: svn://svn.chromium.org/blink/trunk@167767 bbb929c8-8fbe-4397-9dbb-9b2b20218538