DevTools: added Tracing agent

This just adds the protocol description and the test, actual
agent is implemented in the browser.

Related chromium change: https://codereview.chromium.org/18748003

Review URL: https://chromiumcodereview.appspot.com/19005008

git-svn-id: svn://svn.chromium.org/blink/trunk@154997 bbb929c8-8fbe-4397-9dbb-9b2b20218538