[net] Switch TestURLRequestContext to use MockCachingHostResolver.

net/socket_stream/socket_stream.cc

Index: net/socket_stream/socket_stream.cc
diff --git a/net/socket_stream/socket_stream.cc b/net/socket_stream/socket_stream.cc
index 4e0eac1cc59cb2b285e9ba679ce912cdbad94c4e..3a3c69ae93fa3a7b56259e8b13fc4735060a10f9 100644
--- a/net/socket_stream/socket_stream.cc
+++ b/net/socket_stream/socket_stream.cc
@@ -246,11 +246,6 @@ const ProxyServer& SocketStream::proxy_server() const {
   return proxy_info_.proxy_server();
 }
 
-void SocketStream::SetHostResolver(HostResolver* host_resolver) {
-  DCHECK(host_resolver);
-  host_resolver_ = host_resolver;
-}
-
 void SocketStream::SetClientSocketFactory(
     ClientSocketFactory* factory) {
   DCHECK(factory);