Timeline: decouple TimelineFrameModel from TimelineModel

We'd like TimelineFrameModel to be independent from TimelineModel,
since we may build the former based on row trace events.
As a drive by, fix "no live updates" experiment which was broken
in a couple of places, especially WRT displaying frames.

BUG=361045
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/319743002

git-svn-id: svn://svn.chromium.org/blink/trunk@175644 bbb929c8-8fbe-4397-9dbb-9b2b20218538