DevTools: remove Tracing.stopped event from the protocol

Tracing.started event is now always send from the browser process. Tracing.stopped is dropped in favor of dataCollected/tracingComplete events.

BUG=398787

Review URL: https://codereview.chromium.org/518643003

git-svn-id: svn://svn.chromium.org/blink/trunk@181082 bbb929c8-8fbe-4397-9dbb-9b2b20218538