Replace scoped_ptr with unique_ptr in webrtc/api/

But keep #including scoped_ptr.h in .h files, so as not to break
WebRTC users who expect those .h files to give them rtc::scoped_ptr.

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1930463002

Cr-Commit-Position: refs/heads/master@{#12530}
diff --git a/webrtc/api/peerconnectioninterface.h b/webrtc/api/peerconnectioninterface.h
index 59bc160..94d2c00 100644
--- a/webrtc/api/peerconnectioninterface.h
+++ b/webrtc/api/peerconnectioninterface.h
@@ -51,6 +51,7 @@
 #ifndef WEBRTC_API_PEERCONNECTIONINTERFACE_H_
 #define WEBRTC_API_PEERCONNECTIONINTERFACE_H_
 
+#include <memory>
 #include <string>
 #include <utility>
 #include <vector>
@@ -591,14 +592,14 @@
   virtual rtc::scoped_refptr<PeerConnectionInterface> CreatePeerConnection(
       const PeerConnectionInterface::RTCConfiguration& configuration,
       const MediaConstraintsInterface* constraints,
-      rtc::scoped_ptr<cricket::PortAllocator> allocator,
-      rtc::scoped_ptr<DtlsIdentityStoreInterface> dtls_identity_store,
+      std::unique_ptr<cricket::PortAllocator> allocator,
+      std::unique_ptr<DtlsIdentityStoreInterface> dtls_identity_store,
       PeerConnectionObserver* observer) = 0;
 
   virtual rtc::scoped_refptr<PeerConnectionInterface> CreatePeerConnection(
       const PeerConnectionInterface::RTCConfiguration& configuration,
-      rtc::scoped_ptr<cricket::PortAllocator> allocator,
-      rtc::scoped_ptr<DtlsIdentityStoreInterface> dtls_identity_store,
+      std::unique_ptr<cricket::PortAllocator> allocator,
+      std::unique_ptr<DtlsIdentityStoreInterface> dtls_identity_store,
       PeerConnectionObserver* observer) = 0;
 
   virtual rtc::scoped_refptr<MediaStreamInterface>