DevTools: don't send sessionId over the protocol

sessionId is no longer passed in Tracing.started command. We anyway don't support scenario when trace event recording is started in two pages concurrently as in that case only one client will be able to read trace log. So we assume that there is always only one sessionId in the trace log. In case there is more than one session id we will use earliest one and ignore others.

BUG=361045,398787

Review URL: https://codereview.chromium.org/503233004

git-svn-id: svn://svn.chromium.org/blink/trunk@180960 bbb929c8-8fbe-4397-9dbb-9b2b20218538