DevTools: Move usedHeapSize from TimelineEvent into counters.

LayoutTests/http/tests/inspector/websocket/timeline-websocket-event-expected.txt

 Tests the Timeline events for WebSocket
 
 WebSocketCreate Properties:
 {
     counters : <object>
     data : {
         identifier : <number>
         url : ws://127.0.0.1:8880/simple
     }
     frameId : <string>
     stackTrace : <object>
     startTime : <number>
     type : "WebSocketCreate"
-    usedHeapSize : <number>
 }
 WebSocketSendHandshakeRequest Properties:
 {
     counters : <object>
     data : {
         identifier : <number>
     }
     frameId : <string>
     startTime : <number>
     type : "WebSocketSendHandshakeRequest"
-    usedHeapSize : <number>
 }
 WebSocketReceiveHandshakeResponse Properties:
 {
     counters : <object>
     data : {
         identifier : <number>
     }
     frameId : <string>
     startTime : <number>
     type : "WebSocketReceiveHandshakeResponse"
-    usedHeapSize : <number>
 }
 WebSocketDestroy Properties:
 {
     counters : <object>
     data : {
         identifier : <number>
     }
     frameId : <string>
     startTime : <number>
     type : "WebSocketDestroy"
-    usedHeapSize : <number>
 }