Web Inspector: display background events on Timeline
https://bugs.webkit.org/show_bug.cgi?id=108599

Reviewed by Pavel Feldman.

- add optional `thread' field to timeline event;
- if thread field is present, display the event as 'background' (hollow bar / list icon), align to nearest foreground event;
- do not account background events in Frame mode of overview.

* inspector/Inspector.json:
* inspector/InspectorTimelineAgent.cpp:
(WebCore::InspectorTimelineAgent::appendBackgroundThreadRecord):
(WebCore):
* inspector/InspectorTimelineAgent.h:
(InspectorTimelineAgent):
* inspector/front-end/TimelineFrameController.js:
(WebInspector.TimelineFrameController.prototype._addRecord):
* inspector/front-end/TimelinePanel.js:
(WebInspector.TimelineRecordListRow.prototype.update):
(WebInspector.TimelineRecordGraphRow.prototype.update):
* inspector/front-end/TimelinePresentationModel.js:
(WebInspector.TimelinePresentationModel.Record):
(WebInspector.TimelinePresentationModel.insertRetrospecitveRecord):
(WebInspector.TimelinePresentationModel.Record.prototype.get isBackground):
* inspector/front-end/timelinePanel.css:
(.timeline-tree-item.background .timeline-tree-icon):
(.timeline-graph-side.background .timeline-graph-bar):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144039 268f45cc-cd09-0410-ab3c-d52691b4dbfc