DevTools: remove Tracing.started event from the protocol

It was usefule for starting timeline recording via console.timeline. After removing console.timeline/timelineEnd this event doesn't make sense.

BUG=412782

Review URL: https://codereview.chromium.org/614663002

git-svn-id: svn://svn.chromium.org/blink/trunk@182829 bbb929c8-8fbe-4397-9dbb-9b2b20218538