Update PageLoadTiming to use base::Optional

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 a61551d4a402d20a48b138a6b65bbef78dcc2da7..8d44cfca4c9a8ccee795748ffae679fccef5c42f 100644
--- a/components/page_load_metrics/common/page_load_timing.h
+++ b/components/page_load_metrics/common/page_load_timing.h
@@ -5,6 +5,7 @@
 #ifndef COMPONENTS_PAGE_LOAD_METRICS_COMMON_PAGE_LOAD_TIMING_H_
 #define COMPONENTS_PAGE_LOAD_METRICS_COMMON_PAGE_LOAD_TIMING_H_
 
+#include "base/optional.h"
 #include "base/time/time.h"
 #include "third_party/WebKit/public/platform/WebLoadingBehaviorFlag.h"
 
@@ -28,53 +29,52 @@ struct PageLoadTiming {
 
   // 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.
-
   // Time that the first byte of the response is received.
-  base::TimeDelta response_start;
+  base::Optional<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;
+  base::Optional<base::TimeDelta> dom_loading;
 
   // Time immediately before the DOMContentLoaded event is fired.
-  base::TimeDelta dom_content_loaded_event_start;
+  base::Optional<base::TimeDelta> dom_content_loaded_event_start;
 
   // Time immediately before the load event is fired.
-  base::TimeDelta load_event_start;
+  base::Optional<base::TimeDelta> load_event_start;
 
   // Time when the first layout is completed.
-  base::TimeDelta first_layout;
+  base::Optional<base::TimeDelta> first_layout;
 
   // Time when the first paint is performed.
-  base::TimeDelta first_paint;
+  base::Optional<base::TimeDelta> first_paint;
   // Time when the first non-blank text is painted.
-  base::TimeDelta first_text_paint;
+  base::Optional<base::TimeDelta> first_text_paint;
   // Time when the first image is painted.
-  base::TimeDelta first_image_paint;
+  base::Optional<base::TimeDelta> first_image_paint;
   // Time when the first contentful thing (image, text, etc.) is painted.
-  base::TimeDelta first_contentful_paint;
+  base::Optional<base::TimeDelta> first_contentful_paint;
 
   // Time that the document's parser started and stopped parsing main resource
   // content.
-  base::TimeDelta parse_start;
-  base::TimeDelta parse_stop;
+  base::Optional<base::TimeDelta> parse_start;
+  base::Optional<base::TimeDelta> parse_stop;
 
   // Sum of times when the parser is blocked waiting on the load of a script.
   // This duration takes place between parser_start and parser_stop, and thus
-  // must be less than or equal to parser_stop - parser_start.
-  base::TimeDelta parse_blocked_on_script_load_duration;
+  // must be less than or equal to parser_stop - parser_start. Note that this
+  // value may be updated multiple times during the period between parse_start
+  // and parse_stop.
+  base::Optional<base::TimeDelta> parse_blocked_on_script_load_duration;
 
   // Sum of times when the parser is blocked waiting on the load of a script
   // that was inserted from document.write. This duration must be less than or
-  // equal to parse_blocked_on_script_load_duration. Note that some uncommon
-  // cases where scripts are loaded via document.write are not currently covered
-  // by this field. See crbug/600711 for details.
-  base::TimeDelta parse_blocked_on_script_load_from_document_write_duration;
+  // equal to parse_blocked_on_script_load_duration. Note that this value may be
+  // updated multiple times during the period between parse_start and
+  // parse_stop. Note that some uncommon cases where scripts are loaded via
+  // document.write are not currently covered by this field. See crbug/600711
+  // for details.
+  base::Optional<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().