Timeline: control tracing from Timeline panel if Tracing experiment is on

This moves tracing start/stop logic from tracing view to Timeline panel.
The tracing is still only started if tracing experiment is on.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/230803002

git-svn-id: svn://svn.chromium.org/blink/trunk@171253 bbb929c8-8fbe-4397-9dbb-9b2b20218538