Cleanup: Replace MessageQueue pointers with Thread pointers.

This is part of a CL series merging rtc::MessageQueue into rtc::Thread.

Bug: webrtc:9883
Change-Id: I4a1bcd44c9523b6402b3f05b50597bdc2e6615e3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/165345
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30216}

rtc_base/win32_socket_server.cc

@@ -698,7 +698,7 @@ AsyncSocket* Win32SocketServer::CreateAsyncSocket(int family, int type) {
   return nullptr;
 }
 
-void Win32SocketServer::SetMessageQueue(MessageQueue* queue) {
+void Win32SocketServer::SetMessageQueue(Thread* queue) {
   message_queue_ = queue;
 }