Timeline: make hover on event dividers usable

- bring event dividers on top of CPU bars, so it's easier to hover;
- add time since recording started to title.

R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/145643004

git-svn-id: svn://svn.chromium.org/blink/trunk@165835 bbb929c8-8fbe-4397-9dbb-9b2b20218538