Stop using some scoped_ptr features that unique_ptr doesn't have

No operator== that accepts one unique_ptr<T> and one T*. No implicit
conversion to bool. No rtc_make_scoped_ptr function.

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1803833002

Cr-Commit-Position: refs/heads/master@{#12048}

webrtc/base/asyncudpsocket.cc

@@ -37,7 +37,6 @@ AsyncUDPSocket* AsyncUDPSocket::Create(SocketFactory* factory,
 
 AsyncUDPSocket::AsyncUDPSocket(AsyncSocket* socket)
     : socket_(socket) {
-  ASSERT(socket_);
   size_ = BUF_SIZE;
   buf_ = new char[size_];