Fix race condition with InspectorCounters::JSEventListenerCounter

InspectorTimelineAgent::setDOMCounters can be called on a worker thread and
therefore should not access the main-thread only InspectorCounters state.

Review URL: https://codereview.chromium.org/27114002

git-svn-id: svn://svn.chromium.org/blink/trunk@159695 bbb929c8-8fbe-4397-9dbb-9b2b20218538