ozone: Initialize a subsystem only if necessary.

ui/base/cursor/ozone/cursor_factory_ozone.cc

Index: ui/base/cursor/ozone/cursor_factory_ozone.cc
diff --git a/ui/base/cursor/ozone/cursor_factory_ozone.cc b/ui/base/cursor/ozone/cursor_factory_ozone.cc
index 709412e0c8b39d9b19e5611ac7f4789c1f7d5798..51fd7f82b9801278f1c9d310675c05eb5bdda01a 100644
--- a/ui/base/cursor/ozone/cursor_factory_ozone.cc
+++ b/ui/base/cursor/ozone/cursor_factory_ozone.cc
@@ -11,17 +11,21 @@ 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;