Implement layout invalidation tracking in devtools

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

 var initialize_Timeline = function() {
 
 InspectorTest.preloadPanel("timeline");
 
 // Scrub values when printing out these properties in the record or data field.
 InspectorTest.timelinePropertyFormatters = {
     children: "formatAsTypeName",
     endTime: "formatAsTypeName",
     requestId: "formatAsTypeName",
     startTime: "formatAsTypeName",
@@ skipping 262 matching lines @@
     }
     if (record.children().length)
         object["children"] = [];
     if (!record.data() && record instanceof WebInspector.TracingTimelineModel.TraceEventRecord)
         object["data"] = record.traceEvent().args;
     InspectorTest.addObject(object, InspectorTest.timelinePropertyFormatters);
 };
 
 InspectorTest.findFirstTimelineRecord = function(type)
 {
+    return InspectorTest.findTimelineRecord(type, 0);
+}
+
+// Find the (n+1)th timeline record of a specific type.
+InspectorTest.findTimelineRecord = function(type, n)
+{
     var result;
     function findByType(record)
     {
         if (record.type() !== type)
             return false;
-        result = record;
-        return true;
+        if (n === 0) {
+            result = record;
+            return true;
+        }
+        n--;
+        return false;
     }
     InspectorTest.timelineModel().forAllRecords(findByType);
     return result;
 }
 
 InspectorTest.FakeFileReader = function(input, delegate, callback)
 {
     this._delegate = delegate;
     this._callback = callback;
     this._input = input;
@@ skipping 58 matching lines @@
         return this._fileSize;
     },
 
     fileName: function()
     {
         return "fakeFile";
     }
 };
 
 };