Report page load timing information for child frames.

chrome/test/data/page_load_metrics/seamless-iframe.css

+/* Copyright 2017 The Chromium Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+/* To prevent an iframe from painting in the document it is hosted in, we use
+ * the 'seamless' class, which prevents the frame from drawing borders or
+ * scrollbars.
+ */
+iframe.seamless {
+    border: 0px none transparent;
+    overflow: hidden;
+}