Implement layout invalidation tracking in devtools

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

Index: LayoutTests/http/tests/inspector/timeline-test.js
diff --git a/LayoutTests/http/tests/inspector/timeline-test.js b/LayoutTests/http/tests/inspector/timeline-test.js
index d340854654ef387d168b96ca23ff0a09914b0581..d52200438f9f0ec64a6d1efa1a35484d61ffac24 100644
--- a/LayoutTests/http/tests/inspector/timeline-test.js
+++ b/LayoutTests/http/tests/inspector/timeline-test.js
@@ -280,13 +280,23 @@ InspectorTest.printTimelineRecordProperties = function(record)
 
 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;