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

chrome/renderer/page_load_metrics/metrics_render_frame_observer.cc

Index: chrome/renderer/page_load_metrics/metrics_render_frame_observer.cc
diff --git a/chrome/renderer/page_load_metrics/metrics_render_frame_observer.cc b/chrome/renderer/page_load_metrics/metrics_render_frame_observer.cc
index febca3a5958aaec7b26a91ea0478096a01a288ca..d3d150e179e1bb3644035ac01d72510859902fbd 100644
--- a/chrome/renderer/page_load_metrics/metrics_render_frame_observer.cc
+++ b/chrome/renderer/page_load_metrics/metrics_render_frame_observer.cc
@@ -144,6 +144,12 @@ PageLoadTiming MetricsRenderFrameObserver::GetTiming() const {
         base::TimeDelta::FromSecondsD(
             perf.parseBlockedOnScriptExecutionFromDocumentWriteDuration());
   }
+
+  if (perf.authorStyleSheetParseDurationBeforeFCP() > 0.0) {
+    timing.author_style_sheet_parse_duration_before_fcp =
+        base::TimeDelta::FromMicroseconds(
+            perf.authorStyleSheetParseDurationBeforeFCP());
+  }
   return timing;
 }