Fix main frame detection in loading metrics.

tracing/tracing/metrics/v8/runtime_stats_metric_test.html

Index: tracing/tracing/metrics/v8/runtime_stats_metric_test.html
diff --git a/tracing/tracing/metrics/v8/runtime_stats_metric_test.html b/tracing/tracing/metrics/v8/runtime_stats_metric_test.html
index 972517f60c0b9c83835ec689960cf874bc6da61b..046ea2488c78ca59f27434d012a500107303818d 100644
--- a/tracing/tracing/metrics/v8/runtime_stats_metric_test.html
+++ b/tracing/tracing/metrics/v8/runtime_stats_metric_test.html
@@ -60,6 +60,13 @@ tr.b.unittest.testSuite(function() {
       rendererProcess.objects.addSnapshot('ptr', 'loading', 'FrameLoader', 300,
           {isLoadingMainFrame: true, frame: {id_ref: '0xdeadbeef'},
             documentLoaderURL: 'http://example.com'});
+      mainThread.sliceGroup.pushSlice(tr.c.TestUtils.newSliceEx({
+        cat: 'loading',
+        title: 'markAsMainFrame',
+        start: 300,
+        duration: 0.0,
+        args: {frame: '0xdeadbeef'}
+      }));
 
       mainThread.sliceGroup.pushSlice(tr.c.TestUtils.newSliceEx({
         cat: 'v8',