Don't attempt to forward declare StringPiece.

net/base/x509_certificate.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_BASE_X509_CERTIFICATE_H_
 #define NET_BASE_X509_CERTIFICATE_H_
 #pragma once
 
 #include <string.h>
 
@@ skipping 20 matching lines @@
 typedef struct x509_st X509;
 typedef struct x509_store_st X509_STORE;
 #elif defined(USE_NSS)
 // Forward declaration; real one in <cert.h>
 struct CERTCertificateStr;
 #endif
 
 class Pickle;
 
 namespace crypto {
-class StringPiece;
 class RSAPrivateKey;
 }  // namespace crypto
 
 namespace net {
 
 class CRLSet;
 class CertVerifyResult;
 
 typedef std::vector<scoped_refptr<X509Certificate> > CertificateList;
 
@@ skipping 507 matching lines @@
   // (Marked mutable because it's used in a const method.)
   mutable base::Lock verification_lock_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(X509Certificate);
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_X509_CERTIFICATE_H_