Add metrics for style update time before FCP.

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 aafb51546e500fae9e33b16b274000b864b67332..2f7a28886f9d9d28fd8df265953e0c5ab83ec282 100644
--- a/chrome/common/page_load_metrics/page_load_timing.h
+++ b/chrome/common/page_load_metrics/page_load_timing.h
@@ -26,6 +26,9 @@ struct StyleSheetTiming {
   // Total time spent parsing author style sheets, before the first contentful
   // paint.
   base::Optional<base::TimeDelta> author_style_sheet_parse_duration_before_fcp;
+
+  // Time spent in Document::updateStyle before FCP.
+  base::Optional<base::TimeDelta> update_style_duration_before_fcp;
 };
 
 // PageLoadTiming contains timing metrics associated with a page load. Many of