DevTools: Move profileStartTime knowledge into CPUProfileDataModel

driveby: remove relative window mode from FlameChart.

BUG=363976

Review URL: https://codereview.chromium.org/246363005

git-svn-id: svn://svn.chromium.org/blink/trunk@172387 bbb929c8-8fbe-4397-9dbb-9b2b20218538