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

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

Index: LayoutTests/http/tests/inspector/elements-test.js
diff --git a/LayoutTests/http/tests/inspector/elements-test.js b/LayoutTests/http/tests/inspector/elements-test.js
index 69b9e44f7714553b9d8bbb1467002fb2484221af..806834286c66fe9bf30b03b1a03ae1b8c6a73baf 100644
--- a/LayoutTests/http/tests/inspector/elements-test.js
+++ b/LayoutTests/http/tests/inspector/elements-test.js
@@ -492,7 +492,7 @@ InspectorTest.dumpElementsTree = function(rootNode, depth, resultsArray)
     {
         if (!treeItem.root) {
             var expander;
-            if (treeItem.hasChildren) {
+            if (treeItem.isExpandable()) {
                 if (treeItem.expanded)
                     expander = "- ";
                 else
@@ -595,7 +595,7 @@ InspectorTest.expandElementsTree = function(callback)
         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;
             }