TimelinePanel: introduce TimelineModel.Record interface and

and rename original TimelineModel.Record into TimelineModel.RecordImpl

BUG=361045

Review URL: https://codereview.chromium.org/269013004

git-svn-id: svn://svn.chromium.org/blink/trunk@173316 bbb929c8-8fbe-4397-9dbb-9b2b20218538