Timeline: synchronously enable tracing in inspected renderer upon starting Timeline

Timeline may be started from the inspected page, so we need to enable tracing synchronously
to capture whatever happens immediately after timeline is started.

BUG=361045

Review URL: https://codereview.chromium.org/246333004

git-svn-id: svn://svn.chromium.org/blink/trunk@172358 bbb929c8-8fbe-4397-9dbb-9b2b20218538