Refactor CopyNativeEvent and add events_mac.mm to start implementing stubs

ui/events/event_utils.h

Index: ui/events/event_utils.h
diff --git a/ui/events/event_utils.h b/ui/events/event_utils.h
index 9c40ab5fa943386e9b5d550f59b3a2fc1628a052..bb156baa0d408a4a2fb618f547fcef1a3bdc72a5 100644
--- a/ui/events/event_utils.h
+++ b/ui/events/event_utils.h
@@ -84,6 +84,15 @@ EVENTS_EXPORT int GetChangedMouseButtonFlagsFromNative(
 EVENTS_EXPORT gfx::Vector2d GetMouseWheelOffset(
     const base::NativeEvent& native_event);
 
+// Return a copy of |native_event|. Depending on the platform, this copy may
+// need to be deleted with ReleaseCopiedNativeEvent().
+EVENTS_EXPORT base::NativeEvent CopyNativeEvent(
+    const base::NativeEvent& native_event);
+
+// Delete a |native_event| previously created by CopyNativeEvent().
+EVENTS_EXPORT void ReleaseCopiedNativeEvent(
+    const base::NativeEvent& native_event);
+
 // Gets the touch id from a native event.
 EVENTS_EXPORT int GetTouchId(const base::NativeEvent& native_event);