Make member internal::SynchronousMethodCall::e_ a non-pointer.

Bug: None
Change-Id: Id634f1c1ccab8de7862160f30f64b5a5e7a19ae9
Reviewed-on: https://webrtc-review.googlesource.com/c/111080
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25650}

api/proxy.h

@@ -143,7 +143,7 @@ class SynchronousMethodCall : public rtc::MessageData,
  private:
   void OnMessage(rtc::Message*) override;
 
-  std::unique_ptr<rtc::Event> e_;
+  rtc::Event e_;
   rtc::MessageHandler* proxy_;
 };