Use a direct include of strings headers in base/.

base/strings/string_piece.h

 // Copyright (c) 2012 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.
 // Copied from strings/stringpiece.h with modifications
 //
 // A string-like object that points to a sized piece of memory.
 //
 // Functions or methods may use const StringPiece& parameters to accept either
 // a "const char*" or a "string" value that will be implicitly converted to
 // a StringPiece.  The implicit conversion means that it is often appropriate
@@ skipping 13 matching lines @@
 #define BASE_STRINGS_STRING_PIECE_H_
 
 #include <stddef.h>
 
 #include <iosfwd>
 #include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/hash_tables.h"
-#include "base/string16.h"
+#include "base/strings/string16.h"
 
 namespace base {
 
 template <typename STRING_TYPE> class BasicStringPiece;
 typedef BasicStringPiece<std::string> StringPiece;
 typedef BasicStringPiece<string16> StringPiece16;
 
 namespace internal {
 
 // Defines the types, methods, operators, and data members common to both
@@ skipping 397 matching lines @@
 }
 inline size_t hash_value(const base::StringPiece16& sp16) {
   HASH_STRING_PIECE(base::StringPiece16, sp16);
 }
 
 #endif  // COMPILER
 
 }  // namespace BASE_HASH_NAMESPACE
 
 #endif  // BASE_STRINGS_STRING_PIECE_H_