DevTools: support async trace events in Timeline

- build a separate track of async events on each thread once all events
    arrive;
- retain individual begin/end async events among regular events for
    convenience of processing;
- explicitly support blink.console category events so we can handle them
    irregardless of their actual name.
- get rid of WebInspector.TracingTimelineUIUtils.styleForTraceEvent() in
    favor of ...eventStyle().

BUG=398785

Review URL: https://codereview.chromium.org/511433002

git-svn-id: svn://svn.chromium.org/blink/trunk@181240 bbb929c8-8fbe-4397-9dbb-9b2b20218538