DevTools: migrate treeoutline from hasChildren to is/setExpandable().

LayoutTests/http/tests/inspector/elements-test.js

 var initialize_ElementTest = function() {
 
 InspectorTest.preloadPanel("elements");
 
 InspectorTest.inlineStyleSection = function()
 {
     return WebInspector.panels.elements.sidebarPanes.styles.sections[0][0];
 }
 
 InspectorTest.computedStyleSection = function()
@@ skipping 474 matching lines @@
             if (userProperties)
                 userProperties = " [" + userProperties + "]";
         }
         return userProperties;
     }
 
     function print(treeItem, prefix, depth)
     {
         if (!treeItem.root) {
             var expander;
-            if (treeItem.hasChildren) {
+            if (treeItem.isExpandable()) {
                 if (treeItem.expanded)
                     expander = "- ";
                 else
                     expander = "+ ";
             } else
                 expander = "  ";
 
             var userProperties = userPropertyDataDump(treeItem);
             var value = prefix + expander + beautify(treeItem.listItemElement) + userProperties;
             if (treeItem.shadowHostToolbar) {
@@ skipping 82 matching lines @@
 InspectorTest.expandElementsTree = function(callback)
 {
     var expandedSomething = false;
     callback = InspectorTest.safeWrap(callback);
 
     function expand(treeItem)
     {
         var children = treeItem.children();
         for (var i = 0; children && i < children.length; ++i) {
             var child = children[i];
-            if (child.hasChildren && !child.expanded) {
+            if (child.isExpandable() && !child.expanded) {
                 child.expand();
                 expandedSomething = true;
             }
             expand(child);
         }
     }
 
     function onAllNodesAvailable()
     {
         InspectorTest.firstElementsTreeOutline().runPendingUpdates();
@@ skipping 262 matching lines @@
     }
 
     function report(error, result)
     {
         InspectorTest.addResult(idValue + JSON.stringify(result, null, 2));
         callback();
     }
 }
 
 };