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

pdf/pdfium/pdfium_page.h

Index: pdf/pdfium/pdfium_page.h
diff --git a/pdf/pdfium/pdfium_page.h b/pdf/pdfium/pdfium_page.h
index 3cca04c67168992bcd39f377f4107f7e321b7d7a..eee57e0f76c8173ce5e342ab0b50a4ab6262946c 100644
--- a/pdf/pdfium/pdfium_page.h
+++ b/pdf/pdfium/pdfium_page.h
@@ -109,8 +109,8 @@ class PDFiumPage {
   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: