Add NavigationToFirstContentfulPaint UMA for ServiceWorker controlled pages.

components/page_load_metrics/common/page_load_timing.h

Index: components/page_load_metrics/common/page_load_timing.h
diff --git a/components/page_load_metrics/common/page_load_timing.h b/components/page_load_metrics/common/page_load_timing.h
index da1ce89917d0fe1155f25e691bedf35ce2c968da..6a2f164b7ceb513e4b308598d98351a60c4e02cd 100644
--- a/components/page_load_metrics/common/page_load_timing.h
+++ b/components/page_load_metrics/common/page_load_timing.h
@@ -25,6 +25,13 @@ struct PageLoadTiming {
   // Time that the navigation for the associated page was initiated.
   base::Time navigation_start;
 
+  // Null when the page isn't controlled by a Service Worker. Otherwise it is
+  // the time immediately before starting the worker, or if the worker is
+  // available, it is the time immediately before sending the fetch event to
+  // the worker. This time is very close to navigation_start, so we don't use
+  // TimeDelta.
+  base::Time worker_start;
+
   // All TimeDeltas are relative to navigation_start
 
   // Time that the first byte of the response is received.