Delete method MessageQueue::set_socketserver

Instead, make the pointer to the associated socket server a
construction time const, and delete its lock.

Introduces a helper class AutoSocketServerThread for code
(mainly tests) which need a socket server associated with
the current thread.

BUG=webrtc:7501

Review-Url: https://codereview.webrtc.org/2828223002
Cr-Commit-Position: refs/heads/master@{#18047}
diff --git a/webrtc/base/physicalsocketserver_unittest.cc b/webrtc/base/physicalsocketserver_unittest.cc
index de0d485..e2f05e9 100644
--- a/webrtc/base/physicalsocketserver_unittest.cc
+++ b/webrtc/base/physicalsocketserver_unittest.cc
@@ -115,16 +115,15 @@
  protected:
   PhysicalSocketTest()
     : server_(new FakePhysicalSocketServer(this)),
-      scope_(server_.get()),
+      thread_(server_.get()),
       fail_accept_(false),
-      max_send_size_(-1) {
-  }
+      max_send_size_(-1) {}
 
   void ConnectInternalAcceptError(const IPAddress& loopback);
   void WritableAfterPartialWrite(const IPAddress& loopback);
 
   std::unique_ptr<FakePhysicalSocketServer> server_;
-  SocketServerScope scope_;
+  rtc::AutoSocketServerThread thread_;
   bool fail_accept_;
   int max_send_size_;
 };