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

chrome/common/page_load_metrics/page_load_timing.h

Index: chrome/common/page_load_metrics/page_load_timing.h
diff --git a/chrome/common/page_load_metrics/page_load_timing.h b/chrome/common/page_load_metrics/page_load_timing.h
index 223813514ef6aecbe679bfe634dae4eaaad72d3c..aafb51546e500fae9e33b16b274000b864b67332 100644
--- a/chrome/common/page_load_metrics/page_load_timing.h
+++ b/chrome/common/page_load_metrics/page_load_timing.h
@@ -11,6 +11,23 @@
 
 namespace page_load_metrics {
 
+struct StyleSheetTiming {
+  StyleSheetTiming();
+  StyleSheetTiming(const StyleSheetTiming& other);
+  ~StyleSheetTiming();
+
+  bool operator==(const StyleSheetTiming& other) const;
+  bool operator!=(const StyleSheetTiming& other) const {
+    return !(*this == other);
+  }
+
+  bool IsEmpty() const;
+
+  // Total time spent parsing author style sheets, before the first contentful
+  // paint.
+  base::Optional<base::TimeDelta> author_style_sheet_parse_duration_before_fcp;
+};
+
 // PageLoadTiming contains timing metrics associated with a page load. Many of
 // the metrics here are based on the Navigation Timing spec:
 // http://www.w3.org/TR/navigation-timing/.
@@ -98,6 +115,8 @@ struct PageLoadTiming {
   base::Optional<base::TimeDelta>
       parse_blocked_on_script_execution_from_document_write_duration;
 
+  StyleSheetTiming style_sheet_timing;
+
   // If you add additional members, also be sure to update operator==,
   // page_load_metrics_messages.h, and IsEmpty().
 };