DevTools: [tracing timeline] console.timeline based on tracing. Blink part.

The fix needs changes on the both sides, chromium and blink.
Looks like two patches, one the first for blink and the
second for chromium would be enough.

The chromium part could not be landed due to changes in protocol.

1) a new hidden command was added to the protocol
for managing tracing mode of console.timeline.
It should be removed with timeline agent.
2) two new events were introduced. Tracing.started and
Tracing.stopped so we can switch the ui into recording mode.
Unfortunately the backend part doesn't trigger stopped event
 yet because stop command to tracing agent is processing on the browser side.

The mechanics of TracingTimelineModel was switched from callbacks to the event based

the chromium side patch https://codereview.chromium.org/423283004

BUG=398787
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/432433002

git-svn-id: svn://svn.chromium.org/blink/trunk@179290 bbb929c8-8fbe-4397-9dbb-9b2b20218538