TimelinePanel: get rid of lastChildEndTime and cpuTime from TimelineModel.record

BUG=361045
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/246893003

git-svn-id: svn://svn.chromium.org/blink/trunk@172137 bbb929c8-8fbe-4397-9dbb-9b2b20218538