DevTools: add Tracing agent on back-end

We used to have tracing agent as a browser target only, this adds handling
of start command to the back-end and moves the session id logic that was
initialliy implemented in https://codereview.chromium.org/247513002/ to
the tracing agent.

BUG=361045
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/254613002

git-svn-id: svn://svn.chromium.org/blink/trunk@172623 bbb929c8-8fbe-4397-9dbb-9b2b20218538