Add ability to return moved value from FunctorMessageHandler, Optional.

webrtc/base/messagehandler.h

Index: webrtc/base/messagehandler.h
diff --git a/webrtc/base/messagehandler.h b/webrtc/base/messagehandler.h
index 6a3c2ef7402bdf9203ef350dcff740f806d1210d..72c0dc6907950a8e0645f1172f7db9a503b16874 100644
--- a/webrtc/base/messagehandler.h
+++ b/webrtc/base/messagehandler.h
@@ -45,23 +45,13 @@ class FunctorMessageHandler : public MessageHandler {
   }
   const ReturnT& result() const { return result_; }
 
- private:
-  FunctorT functor_;
-  ReturnT result_;
-};
-
-// Specialization for std::unique_ptr<ReturnT>.
-template <class ReturnT, class FunctorT>
-class FunctorMessageHandler<class std::unique_ptr<ReturnT>, FunctorT>
-    : public MessageHandler {
- public:
-  explicit FunctorMessageHandler(const FunctorT& functor) : functor_(functor) {}
-  virtual void OnMessage(Message* msg) { result_ = std::move(functor_()); }
-  std::unique_ptr<ReturnT> result() { return std::move(result_); }
+  // Returns moved result. Should not call result() or MoveResult() again
+  // after this.
+  ReturnT MoveResult() { return std::move(result_); }
 
  private:
   FunctorT functor_;
-  std::unique_ptr<ReturnT> result_;
+  ReturnT result_;
 };
 
 // Specialization for ReturnT of void.
@@ -74,6 +64,7 @@ class FunctorMessageHandler<void, FunctorT> : public MessageHandler {
     functor_();
   }
   void result() const {}
+  void MoveResult() {}
 
  private:
   FunctorT functor_;