PageLoadTiming fixes in prep for using 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..10726f075f222a9049ba43401de300eb9f986ccf 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"
 
@@ -76,6 +77,8 @@ struct PageLoadTiming {
   // by this field. See crbug/600711 for details.
   base::TimeDelta parse_blocked_on_script_load_from_document_write_duration;
 
+  base::Optional<base::TimeDelta> testing;
+
   // If you add additional members, also be sure to update operator==,
   // page_load_metrics_messages.h, and IsEmpty().
 };