Introduce TracingTimelineModel

TimelineTraceEventBindings is renamed to TracingTimelineModel. All logic related to timeline-specific trace events will be moved from TracingModel into TracingTimelineModel. TracingModel will be a lower level model which works with generic trace events.

In the next patch I'm going to move inspectedTargetEvents to TracingTimelineModel.

BUG=361045
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/310313003

git-svn-id: svn://svn.chromium.org/blink/trunk@175448 bbb929c8-8fbe-4397-9dbb-9b2b20218538