DevTools: tracing: remove state management from InspectorTracingAgent.

Tracing state management happens on the browser side. So the local state does nothing.
Also if DevTools window was closed when tracing was active the next DevTools session had
this flag enabled. So it was not possible to correctly process tracing data because they
had no TracingStartedInPage event. The state used to be necessary when we had console.timeline/timelineEnd implementation.

This test case has been covered by timeline-trivial.html

BUG=420008
R= yurys@chromium.org, caseq@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/638673002

git-svn-id: svn://svn.chromium.org/blink/trunk@183608 bbb929c8-8fbe-4397-9dbb-9b2b20218538