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

chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
index 5c0d8a07fd842e80d6f16fb3970abb6f3d4b224e..0f673ca2900bf435c72254cb33eea06017c8d356 100644
--- a/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
@@ -332,6 +332,14 @@ void CorePageLoadMetricsObserver::OnFirstContentfulPaint(
                           timing.first_contentful_paint.value());
     }
 
+    if (timing.author_style_sheet_parse_duration_before_fcp) {
+      UMA_HISTOGRAM_CUSTOM_COUNTS(
+          "PageLoad.CSSTiming.Parse.BeforeFirstContentfulPaint",
+          timing.author_style_sheet_parse_duration_before_fcp.value()
+              .InMicroseconds(),
+          1, base::TimeDelta::FromSeconds(10).InMicroseconds(), 50);
+    }
+
     switch (GetPageLoadType(transition_)) {
       case LOAD_TYPE_RELOAD:
         PAGE_LOAD_HISTOGRAM(