Don't attempt to forward declare StringPiece.

net/dns/dns_client.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_DNS_DNS_CLIENT_H_
 #define NET_DNS_DNS_CLIENT_H_
 #pragma once
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/memory/weak_ptr.h"
+#include "base/string_piece.h"
 #include "net/base/net_export.h"
 
-namespace base {
-class StringPiece;
-}
-
 namespace net {
 
 class BoundNetLog;
 class ClientSocketFactory;
 class DnsResponse;
 class DnsSession;
 
 // DnsClient performs asynchronous DNS queries. DnsClient takes care of
 // retransmissions, DNS server fallback (or round-robin), suffix search, and
 // simple response validation ("does it match the query") to fight poisoning.
@@ skipping 54 matching lines @@
       const BoundNetLog& source_net_log) WARN_UNUSED_RESULT = 0;
 
   // Creates a socket-based DnsClient using the |session|.
   static DnsClient* CreateClient(DnsSession* session) WARN_UNUSED_RESULT;
 };
 
 }  // namespace net
 
 #endif  // NET_DNS_DNS_CLIENT_H_