Tests the Timeline API instrumentation of a DOM Dispatch (mousedown)

Test Mouse Target
EventDispatch Properties:
{
    children : <object>
    counters : <object>
    data : {
        type : "mousedown"
    }
    endTime : <number>
    frameId : <string>
    startTime : <number>
    type : "EventDispatch"
    usedHeapSize : <number>
}