DevTools: Rename WebInspector.DOMAgent into WebInspector.DOMModel

LayoutTests/inspector/elements/resources/iframe-load-event-iframe.js

 function loadSecondIFrame()
 {
     document.getElementById("myframe").onload = null;
     document.getElementById("myframe").src = "resources/iframe-load-event-iframe-2.html";
 }
 
 function test()
 {
     InspectorTest.expandElementsTree(step1);
 
     function step1()
     {
-        WebInspector.domAgent.addEventListener(WebInspector.DOMAgent.Events.NodeInserted, nodeInserted);
+        WebInspector.domModel.addEventListener(WebInspector.DOMModel.Events.NodeInserted, nodeInserted);
         InspectorTest.evaluateInPage("loadSecondIFrame()");
 
         function nodeInserted(event)
         {
             var node = event.data;
             if (node.getAttribute("id") === "myframe") {
                 InspectorTest.expandElementsTree(step2);
-                WebInspector.domAgent.removeEventListener(WebInspector.DOMAgent.Events.NodeInserted, nodeInserted);
+                WebInspector.domModel.removeEventListener(WebInspector.DOMModel.Events.NodeInserted, nodeInserted);
             }
         }
     }
 
     function step2()
     {
         InspectorTest.addResult("\n\nAfter frame navigate");
         InspectorTest.dumpElementsTree();
         InspectorTest.completeTest();
     }
 }