Extract implementation based on timeline events into TimelineModelImpl

We will have two descendants of TimelineModel - one based on trace events and another one based on the timeline agent events. This change extracts the latter one.

I left WebInspector.TimelineModel.RecordType to keep the patch short. Will move it to WebInspector.TimelineModelImpl.RecordType later.

BUG=361045
R=caseq@chromium.org

Review URL: https://codereview.chromium.org/313333002

git-svn-id: svn://svn.chromium.org/blink/trunk@175577 bbb929c8-8fbe-4397-9dbb-9b2b20218538