DevTools: Speed up timeline aggregated tree building.

third_party/WebKit/LayoutTests/http/tests/inspector/timeline-test.js

 function wrapCallFunctionForTimeline(f)
 {
     var script = document.createElement("script");
     script.textContent = "(" + f.toString() + ")()\n//# sourceURL=wrapCallFunctionForTimeline.js";
     document.body.appendChild(script);
 }
 
 var initialize_Timeline = function() {
 
 InspectorTest.preloadPanel("timeline");
@@ skipping 208 matching lines @@
     var selection = Timeline.TimelineSelection.fromRange(model.minimumRecordTime(), model.maximumRecordTime());
     view.updateContents(selection);
     InspectorTest.walkTimelineEventTreeUnderNode(callback, view._currentTree, 0);
 }
 
 InspectorTest.walkTimelineEventTreeUnderNode = function(callback, root, level)
 {
     var event = root.event;
     if (event)
         callback(event, level)
-    var children = root.children ? root.children.values() : [];
-    for (var child of children)
+    for (var child of root.children().values())
         InspectorTest.walkTimelineEventTreeUnderNode(callback, child, (level || 0) + 1);
 }
 
 InspectorTest.printTimestampRecords = function(typeName)
 {
     var dividers = InspectorTest.timelineModel().eventDividers();
     for (var event of dividers) {
         if (event.name === typeName)
             InspectorTest.printTraceEventProperties(event);
     }
@@ skipping 201 matching lines @@
 function waitForFrame()
 {
     var callback;
     var promise = new Promise((fulfill) => callback = fulfill);
     if (window.testRunner)
         testRunner.capturePixelsAsyncThen(() => window.requestAnimationFrame(callback));
     else
         window.requestAnimationFrame(callback);
     return promise;
 }