DevTools: log worker thread ids on the main thread when Timeline recording is enabled

DevTools uses TracingStartedInWorker trace event to figure out which worker threads correspond to the inspected page. The events were written on the worker thread which is racy. Current implementation always writes the event on the main thread. Since Tracing.start/end commands are handled on the main thread this eliminates racy behavior.

BUG=401895,405429

Review URL: https://codereview.chromium.org/552323002

git-svn-id: svn://svn.chromium.org/blink/trunk@181820 bbb929c8-8fbe-4397-9dbb-9b2b20218538