Optional <TimeDelta> since they may be non existent.

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 285cf65aa3b946cf78d7c4ed430ca69ac6fcf65b..aad5c71b4f5b043eec9ed5564af5e1e9a6e51913 100644
--- a/components/page_load_metrics/common/page_load_timing.h
+++ b/components/page_load_metrics/common/page_load_timing.h
@@ -26,6 +26,13 @@ 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 when serialization
+  // supports it.
+
   // Time that the first byte of the response is received.
   base::TimeDelta response_start;