Add CSSTiming to collect aggregate PLT-level stats about CSS.

chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc

 // Copyright (c) 2015 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.
 
 #include "base/files/scoped_temp_dir.h"
 #include "base/macros.h"
 #include "base/test/histogram_tester.h"
 #include "base/time/time.h"
 #include "chrome/browser/page_load_metrics/metrics_web_contents_observer.h"
 #include "chrome/browser/page_load_metrics/observers/aborts_page_load_metrics_observer.h"
@@ skipping 518 matching lines @@
 
   histogram_tester_.ExpectTotalCount(
       prerender::PrerenderHistograms::GetFirstContentfulPaintHistogramName(
           prerender::ORIGIN_NONE, false, true, base::TimeDelta()),
       1);
   histogram_tester_.ExpectTotalCount(
       prerender::PrerenderHistograms::GetFirstContentfulPaintHistogramName(
           prerender::ORIGIN_NONE, false, false, base::TimeDelta()),
       0);
 }
+
+IN_PROC_BROWSER_TEST_F(PageLoadMetricsBrowserTest, CSSTiming) {
+  ASSERT_TRUE(embedded_test_server()->Start());
+
+  // Careful: Blink code clamps timestamps to 5us, so any CSS parsing we do here
+  // must take >> 5us, otherwise we'll log 0 for the value and it will remain
+  // unset here.
+  ui_test_utils::NavigateToURL(
+      browser(),
+      embedded_test_server()->GetURL("/page_load_metrics/page_with_css.html"));
+  NavigateToUntrackedUrl();
+
+  histogram_tester_.ExpectTotalCount(internal::kHistogramFirstContentfulPaint,
+                                     1);
+  histogram_tester_.ExpectTotalCount(
+      "PageLoad.CSSTiming.Parse.BeforeFirstContentfulPaint", 1);
+}