Don't create TimelineModel.RecordImpl in TimelinePresentationModel.js

TimelinePresentationModel.Record now has three implementations: one for root record, one for coealesced record and one for representing actual TimelineModel.Record. This allows it to work fine with Timeline based on trace events.

BUG=361045
R=caseq@chromium.org

Review URL: https://codereview.chromium.org/337023004

git-svn-id: svn://svn.chromium.org/blink/trunk@176205 bbb929c8-8fbe-4397-9dbb-9b2b20218538