DevTools: Move usedHeapSize from TimelineEvent into counters.

LayoutTests/inspector/timeline/timeline-dom-content-loaded-event-expected.txt

 Tests the DOM content loaded event.
 
 Page reloaded.
 MarkDOMContent Properties:
 {
     counters : <object>
     data : {
         isMainFrame : true
     }
     frameId : <string>
     startTime : <number>
     type : "MarkDOMContent"
-    usedHeapSize : <number>
 }