TimelinePanel: remove unnecessary fields, getters and setters from TimelineModel.Records

model field getter was removed
isRoot field getter was removed
startTime field setter was removed
_startTimeOffset field was removed
calculateAggregatedStats was marked as private
addWarning was marked as private

BUG=361045
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/247653002

git-svn-id: svn://svn.chromium.org/blink/trunk@172346 bbb929c8-8fbe-4397-9dbb-9b2b20218538