Fix main frame detection in loading metrics.

tracing/tracing/metrics/system_health/power_metric_test.html

Index: tracing/tracing/metrics/system_health/power_metric_test.html
diff --git a/tracing/tracing/metrics/system_health/power_metric_test.html b/tracing/tracing/metrics/system_health/power_metric_test.html
index c2ce6089152944bfd18af011e7e0d53ff3715c7a..6758beba51c331ee9244dc38013a53937153f9e9 100644
--- a/tracing/tracing/metrics/system_health/power_metric_test.html
+++ b/tracing/tracing/metrics/system_health/power_metric_test.html
@@ -451,6 +451,13 @@ 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: 'loading',