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

ui/events/event_rewriter_unittest.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/events/event_rewriter.h"
 
 #include <list>
 #include <map>
 #include <set>
 #include <utility>
 
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/events/test/test_event_processor.h"
 
 namespace ui {
 
 namespace {
 
 // To test the handling of |EventRewriter|s through |EventSource|,
 // we rewrite and test event types.
 class TestEvent : public Event {
  public:
   explicit TestEvent(EventType type)
       : Event(type, base::TimeDelta(), 0), unique_id_(next_unique_id_++) {}
   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_;
 };
 
 int TestEvent::next_unique_id_ = 0;
 
 // TestEventRewriteProcessor is set up with a sequence of event types,
 // and fails if the events received via OnEventFromSource() do not match
 // this sequence. These expected event types are consumed on receipt.
@@ skipping 184 matching lines @@
   s.RemoveEventRewriter(&r3);
 
   // Continue with the state-based rewriting.
   p.AddExpectedEvent(ET_MOUSE_RELEASED);
   p.AddExpectedEvent(ET_KEY_RELEASED);
   s.Send(ET_MOUSE_RELEASED);
   p.CheckAllReceived();
 }
 
 }  // namespace ui