Do not mutate ui::Event properties during nested event processing

ui/events/event_rewriter_unittest.cc

Index: ui/events/event_rewriter_unittest.cc
diff --git a/ui/events/event_rewriter_unittest.cc b/ui/events/event_rewriter_unittest.cc
index 11a05c9c1572df48b385276d4ea8e4fd623a8283..6af9519728626c0d247dda2372b6db2e1ee009e1 100644
--- a/ui/events/event_rewriter_unittest.cc
+++ b/ui/events/event_rewriter_unittest.cc
@@ -25,6 +25,11 @@ class TestEvent : public Event {
   virtual ~TestEvent() {}
   int unique_id() const { return unique_id_; }
 
+  // Event:
+  virtual scoped_ptr<Event> Clone() const OVERRIDE {
+    return scoped_ptr<Event>(new TestEvent(*this));
+  }
+
  private:
   static int next_unique_id_;
   int unique_id_;