DevTools: fix aggegated pie chart for JSFrame events on Timeline

JSFrame events are added in TimelineFlameChart and their selfTime is never calculated. These events are never added into the model as they are used only to draw the flame chart. When such event is selected we calculate its selfTime on the fly as duration - sum of children times.

BUG=424507

Review URL: https://codereview.chromium.org/645673004

git-svn-id: svn://svn.chromium.org/blink/trunk@183893 bbb929c8-8fbe-4397-9dbb-9b2b20218538