Show markers on Timeline overview in tracing based Timeline

TimelinePanel._addRecord was not called for divider events, on the other hand there is an explicit list of divider events exposed on TimelineModel.

BUG=399531
R=alph@chromium.org

Review URL: https://codereview.chromium.org/441803006

git-svn-id: svn://svn.chromium.org/blink/trunk@179601 bbb929c8-8fbe-4397-9dbb-9b2b20218538