Fix flakiness

__tests__/html/timestamp.attachmentSendTimeout.html

@@ -65,6 +65,7 @@
 
           // WHEN: Sending out the file.
           await pageObjects.runHook(({ useSendFiles }) => useSendFiles()([fileBlob]));
+          await pageConditions.numActivitiesShown(2);
 
           // THEN: It should show "Sending".
           //       Note the first activity is reserved for "run hook", we are checking activity status from the second activity.