Remove aggregatedStats from TimelineModel.Record

Time statistics aggregated by category is calculated on the fly, no need to store it on the model records.

BUG=361045
R=caseq@chromium.org

Review URL: https://codereview.chromium.org/341093005

git-svn-id: svn://svn.chromium.org/blink/trunk@176604 bbb929c8-8fbe-4397-9dbb-9b2b20218538