ozone: Initialize a subsystem only if necessary.

ui/events/ozone/event_factory_ozone.cc

Index: ui/events/ozone/event_factory_ozone.cc
diff --git a/ui/events/ozone/event_factory_ozone.cc b/ui/events/ozone/event_factory_ozone.cc
index 0cc699e04ebe334d7bbd1167d522b40febb2f4c1..3a56ddc968fefcfac468ba4e22bd72c7b8b4316c 100644
--- a/ui/events/ozone/event_factory_ozone.cc
+++ b/ui/events/ozone/event_factory_ozone.cc
@@ -11,17 +11,21 @@ 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,