Optional <TimeDelta> since they may be non existent.

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();
 
   bool operator==(const PageLoadTiming& other) const;
 
   bool IsEmpty() const;
 
   // Time that the navigation for the associated page was initiated.
   base::Time navigation_start;
 
   // All TimeDeltas are relative to navigation_start
 
+  // @shivanisha TODO Issue 596367 shows that it is possible for a existent

[Bryan McQuade, 2016/03/29 18:33:43]

style nit:
TODO(shivanisha): Issue ...

[shivanisha, 2016/03/29 18:54:25]

updating the comment

+  // TimeDelta value to be 0 (2 TimeTicks can have the same value even if they
+  // were assigned in separate instructions if the clock speed is less
+  // granular). The solution there was to use base::Optional for those values.
+  // Consider changing the below values to Optional as well when serialization
+  // supports it.
+
   // 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.
   base::TimeDelta dom_content_loaded_event_start;
 
@@ skipping 12 matching lines @@
   // Time when the first contentful thing (image, text, etc.) is painted.
   base::TimeDelta first_contentful_paint;
 
   // 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_