Add NavigationToFirstContentfulPaint UMA for ServiceWorker controlled pages.

components/page_load_metrics/common/page_load_timing.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_PAGE_LOAD_METRICS_COMMON_PAGE_LOAD_TIMING_H_
 #define COMPONENTS_PAGE_LOAD_METRICS_COMMON_PAGE_LOAD_TIMING_H_
 
 #include "base/time/time.h"
 
 namespace page_load_metrics {
 
 // 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/.
 struct PageLoadTiming {
  public:
   PageLoadTiming();
   PageLoadTiming(const PageLoadTiming& other);
   ~PageLoadTiming();
 
   bool operator==(const PageLoadTiming& other) const;
 
   bool IsEmpty() const;
 
   // 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.
   base::TimeDelta response_start;
 
   // Time that the document transitions to the 'loading' state. This is roughly
   // the time that the HTML parser begins parsing the main HTML resource.
   base::TimeDelta dom_loading;
 
   // Time immediately before the DOMContentLoaded event is fired.
@@ skipping 31 matching lines @@
   // by this field. See crbug/600711 for details.
   base::TimeDelta parse_blocked_on_script_load_from_document_write_duration;
 
   // If you add additional members, also be sure to update operator==,
   // page_load_metrics_messages.h, and IsEmpty().
 };
 
 }  // namespace page_load_metrics
 
 #endif  // COMPONENTS_PAGE_LOAD_METRICS_COMMON_PAGE_LOAD_TIMING_H_