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

chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc

Index: chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
diff --git a/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc b/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
index 285640c1d2d01675c476d5e5e6312ed17ccf301a..51e84954d9a08b0d6bda068bb87da704b53ac6df 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
+++ b/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
@@ -633,3 +633,18 @@ IN_PROC_BROWSER_TEST_F(PageLoadMetricsBrowserTest,
           prerender::ORIGIN_NONE, false, false, base::TimeDelta()),
       0);
 }
+
+IN_PROC_BROWSER_TEST_F(PageLoadMetricsBrowserTest, CSSTiming) {
+  ASSERT_TRUE(embedded_test_server()->Start());
+
+  ui_test_utils::NavigateToURL(
+      browser(),
+      embedded_test_server()->GetURL("/page_load_metrics/page_with_css.html"));
+  NavigateToUntrackedUrl();
+
+  WaitForTimingUpdatedIPC();
+  histogram_tester_.ExpectTotalCount(internal::kHistogramFirstContentfulPaint,
+                                     1);
+  histogram_tester_.ExpectTotalCount(
+      "PageLoad.CSSTiming.Parse.BeforeFirstContentfulPaint", 1);
+}