Refactor CopyNativeEvent and add events_mac.mm to start implementing stubs

ui/events/ozone/events_ozone.cc

Index: ui/events/ozone/events_ozone.cc
diff --git a/ui/events/ozone/events_ozone.cc b/ui/events/ozone/events_ozone.cc
index 2ffc56a41d91764c49a4885d68c8639759d22de8..0ec86189e07b3434d2f3f0204cd09238af70a104 100644
--- a/ui/events/ozone/events_ozone.cc
+++ b/ui/events/ozone/events_ozone.cc
@@ -65,6 +65,12 @@ gfx::Vector2d GetMouseWheelOffset(const base::NativeEvent& native_event) {
   return event->offset();
 }
 
+base::NativeEvent CopyNativeEvent(const base::NativeEvent& event) {
+  return NULL;
+}
+
+void ReleaseCopiedNativeEvent(const base::NativeEvent& event) {}
+
 void ClearTouchIdIfReleased(const base::NativeEvent& xev) {
 }