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 7e1f83fb8ecbc0a7b48444138e663470dde55741..256ddba854aee8d0dc7ba2d4beaf89a9c061b50a 100644
--- a/ui/events/ozone/event_factory_ozone.cc
+++ b/ui/events/ozone/event_factory_ozone.cc
@@ -15,17 +15,21 @@ 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,