Don't attempt to forward declare StringPiece.

net/socket/ssl_socket.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_SOCKET_SSL_SOCKET_H_
 #define NET_SOCKET_SSL_SOCKET_H_
 
 #include "base/basictypes.h"
+#include "base/string_piece.h"
 #include "net/socket/stream_socket.h"
 
-namespace base {
-class StringPiece;
-}  // namespace base
-
 namespace net {
 
 // SSLSocket interface defines method that are common between client
 // and server SSL sockets.
 class NET_EXPORT SSLSocket : public StreamSocket {
 public:
   virtual ~SSLSocket() {}
 
   // Exports data derived from the SSL master-secret (see RFC 5705).
   // The call will fail with an error if the socket is not connected, or the
   // SSL implementation does not support the operation.
   virtual int ExportKeyingMaterial(const base::StringPiece& label,
                                    const base::StringPiece& context,
                                    unsigned char *out,
                                    unsigned int outlen) = 0;
 };
 
 }  // namespace net
 
 #endif  // NET_SOCKET_SSL_SOCKET_H_