event_sender should include a timestamp when firing keyboard events

BUG=160524

Review URL: https://codereview.chromium.org/1353943002

Cr-Commit-Position: refs/heads/master@{#349475}

Author: majido

components/test_runner/event_sender.cc

@@ -1396,6 +1396,7 @@ void EventSender::KeyDown(const std::string& code_str,
   // Windows event flow; on other platforms we create a merged event and test
   // the event flow that that platform provides.
   WebKeyboardEvent event_down;
+  event_down.timeStampSeconds = GetCurrentEventTimeSec();
   event_down.type = WebInputEvent::RawKeyDown;
   event_down.modifiers = modifiers;
   event_down.windowsKeyCode = code;