ozone: Initialize a subsystem only if necessary.

ui/base/cursor/ozone/cursor_factory_ozone.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/base/cursor/ozone/cursor_factory_ozone.h"
 
 #include "base/logging.h"
 
 namespace ui {
 
 // static
 CursorFactoryOzone* CursorFactoryOzone::impl_ = NULL;
 
-CursorFactoryOzone::CursorFactoryOzone() {}
+CursorFactoryOzone::CursorFactoryOzone() {
+  CHECK(!impl_) << "There should only be a single CursorFactoryOzone.";
+  impl_ = this;
+}
 
-CursorFactoryOzone::~CursorFactoryOzone() {}
+CursorFactoryOzone::~CursorFactoryOzone() {
+  CHECK_EQ(impl_, this);
+  impl_ = NULL;
+}
 
 CursorFactoryOzone* CursorFactoryOzone::GetInstance() {
   CHECK(impl_) << "No CursorFactoryOzone implementation set.";
   return impl_;
 }
 
-void CursorFactoryOzone::SetInstance(CursorFactoryOzone* impl) { impl_ = impl; }
-
 PlatformCursor CursorFactoryOzone::GetDefaultCursor(int type) {
   NOTIMPLEMENTED();
   return NULL;
 }
 
 PlatformCursor CursorFactoryOzone::CreateImageCursor(
     const SkBitmap& bitmap,
     const gfx::Point& hotspot) {
   NOTIMPLEMENTED();
   return NULL;
@@ skipping 11 matching lines @@
                                    PlatformCursor cursor) {
   NOTIMPLEMENTED();
 }
 
 gfx::AcceleratedWidget CursorFactoryOzone::GetCursorWindow() {
   NOTIMPLEMENTED();
   return 0;
 }
 
 }  // namespace ui