Use a direct include of strings headers in ppapi/, printing/.

printing/backend/print_backend.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.
 
 #ifndef PRINTING_BACKEND_PRINT_BACKEND_H_
 #define PRINTING_BACKEND_PRINT_BACKEND_H_
 
 #include <map>
 #include <string>
 #include <vector>
 
 #include "base/memory/ref_counted.h"
-#include "base/string16.h"
+#include "base/strings/string16.h"
 #include "printing/print_job_constants.h"
 #include "printing/printing_export.h"
 
 namespace base {
 class DictionaryValue;
 }
 
 // This is the interface for platform-specific code for a print backend
 namespace printing {
 
@@ skipping 79 matching lines @@
       const base::DictionaryValue* print_backend_settings);
 
  protected:
   friend class base::RefCountedThreadSafe<PrintBackend>;
   virtual ~PrintBackend();
 };
 
 }  // namespace printing
 
 #endif  // PRINTING_BACKEND_PRINT_BACKEND_H_