Cleanup: Pass std::string as const reference from pdf/

pdf/pdfium/pdfium_page.h

 // Copyright (c) 2010 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 PDF_PDFIUM_PDFIUM_PAGE_H_
 #define PDF_PDFIUM_PDFIUM_PAGE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 91 matching lines @@
   void CalculateLinks();
   // Returns link type and target associated with a link. Returns
   // NONSELECTABLE_AREA if link detection failed.
   Area GetLinkTarget(FPDF_LINK link, LinkTarget* target);
   // Returns target associated with a destination.
   Area GetDestinationTarget(FPDF_DEST destination, LinkTarget* target);
   // Returns the text in the supplied box as a Value Node
   base::Value* GetTextBoxAsValue(double page_height, double left, double top,
                                  double right, double bottom, int rotation);
   // Helper functions for JSON generation
-  base::Value* CreateTextNode(std::string text);
-  base::Value* CreateURLNode(std::string text, std::string url);
+  base::Value* CreateTextNode(const std::string& text);
+  base::Value* CreateURLNode(const std::string& text, const std::string& url);
 
   class ScopedLoadCounter {
    public:
     explicit ScopedLoadCounter(PDFiumPage* page);
     ~ScopedLoadCounter();
 
    private:
     PDFiumPage* const page_;
   };
 
@@ skipping 13 matching lines @@
   int loading_count_;
   pp::Rect rect_;
   bool calculated_links_;
   std::vector<Link> links_;
   bool available_;
 };
 
 }  // namespace chrome_pdf
 
 #endif  // PDF_PDFIUM_PDFIUM_PAGE_H_