Rewrite firstPaintMetric to support extracting multiple navigations from a single trace

tracing/tracing/model/object_instance.html

Index: tracing/tracing/model/object_instance.html
diff --git a/tracing/tracing/model/object_instance.html b/tracing/tracing/model/object_instance.html
index cfc1356321feb70c6e1535e6861a389bd3d7d425..37337712073ec63d18e1f63c2dc5203ac62a705d 100644
--- a/tracing/tracing/model/object_instance.html
+++ b/tracing/tracing/model/object_instance.html
@@ -6,10 +6,10 @@ found in the LICENSE file.
 -->
 
 <link rel="import" href="/tracing/base/extension_registry.html">
-<link rel="import" href="/tracing/model/event.html">
-<link rel="import" href="/tracing/model/object_snapshot.html">
 <link rel="import" href="/tracing/base/range.html">
 <link rel="import" href="/tracing/base/sorted_array_utils.html">
+<link rel="import" href="/tracing/model/event.html">
+<link rel="import" href="/tracing/model/object_snapshot.html">
 
 <script>
 'use strict';
@@ -120,6 +120,15 @@ tr.exportTo('tr.model', function() {
         this.snapshots[i].initialize();
     },
 
+    isAliveAt: function(ts) {
+      if (ts < this.creationTs && this.creationTsWasExplicit)
+        return false;
+      if (ts > this.deletionTs)
+        return false;
+
+      return true;
+    },
+
     getSnapshotAt: function(ts) {
       if (ts < this.creationTs) {
         if (this.creationTsWasExplicit)