ozone: Initialize a subsystem only if necessary.

ui/events/ozone/event_factory_ozone.cc

 // Copyright (c) 2013 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/ozone/event_factory_ozone.h"
 
 #include "base/logging.h"
 
 namespace ui {
 
 // static
 EventFactoryOzone* EventFactoryOzone::impl_ = NULL;
 
-EventFactoryOzone::EventFactoryOzone() {}
+EventFactoryOzone::EventFactoryOzone() {
+  CHECK(!impl_) << "There should only be a single EventFactoryOzone";
+  impl_ = this;
+}
 
-EventFactoryOzone::~EventFactoryOzone() {}
+EventFactoryOzone::~EventFactoryOzone() {
+  CHECK_EQ(impl_, this);
+  impl_ = NULL;
+}
 
 EventFactoryOzone* EventFactoryOzone::GetInstance() {
   CHECK(impl_) << "No EventFactoryOzone implementation set.";
   return impl_;
 }
 
-void EventFactoryOzone::SetInstance(EventFactoryOzone* impl) { impl_ = impl; }
-
 void EventFactoryOzone::StartProcessingEvents() {}
 
 void EventFactoryOzone::WarpCursorTo(gfx::AcceleratedWidget widget,
                                      const gfx::PointF& location) {
   NOTIMPLEMENTED();
 }
 
 }  // namespace ui