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"
 #include "third_party/WebKit/public/platform/WebLoadingBehaviorFlag.h"
 
@@ skipping 10 matching lines @@
 
   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
 
+  // TODO(shivanisha): Issue 596367 shows that it is possible for a valid
+  // 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 36 matching lines @@
 struct PageLoadMetadata {
   PageLoadMetadata();
   bool operator==(const PageLoadMetadata& other) const;
   // These are packed blink::WebLoadingBehaviorFlag enums.
   int behavior_flags = blink::WebLoadingBehaviorNone;
 };
 
 }  // namespace page_load_metrics
 
 #endif  // COMPONENTS_PAGE_LOAD_METRICS_COMMON_PAGE_LOAD_TIMING_H_