Make scheduler into unique_ptr instead of shared_ptr

Summary:
Changelog: [Internal]

RCTScheduler was storing Scheduler as `shared_ptr`, but `RCTScheduler` is sole owner of it.
`unique_ptr` better expresses this ownership.

Reviewed By: JoshuaGross

Differential Revision: D21923573

fbshipit-source-id: e382f2d6e0a4875e1441b6063c1ad7056b338e29

React/Fabric/RCTScheduler.mm

@@ -95,7 +95,7 @@ class LayoutAnimationDelegateProxy : public LayoutAnimationStatusDelegate, publi
 };
 
 @implementation RCTScheduler {
-  std::shared_ptr<Scheduler> _scheduler;
+  std::unique_ptr<Scheduler> _scheduler;
   std::shared_ptr<LayoutAnimationDriver> _animationDriver;
   std::shared_ptr<SchedulerDelegateProxy> _delegateProxy;
   std::shared_ptr<LayoutAnimationDelegateProxy> _layoutAnimationDelegateProxy;
@@ -120,7 +120,7 @@ class LayoutAnimationDelegateProxy : public LayoutAnimationStatusDelegate, publi
       _uiRunLoopObserver->setDelegate(_layoutAnimationDelegateProxy.get());
     }
 
-    _scheduler = std::make_shared<Scheduler>(
+    _scheduler = std::make_unique<Scheduler>(
         toolbox, (_animationDriver ? _animationDriver.get() : nullptr), _delegateProxy.get());
   }
 
@@ -138,8 +138,6 @@ class LayoutAnimationDelegateProxy : public LayoutAnimationStatusDelegate, publi
     _animationDriver->setLayoutAnimationStatusDelegate(nullptr);
   }
   _animationDriver = nullptr;
-
-  _scheduler->setDelegate(nullptr);
 }
 
 - (void)startSurfaceWithSurfaceId:(SurfaceId)surfaceId