Cleanup: Pass std::string as const reference if possible

ios/chrome/common/string_util.h

 // Copyright 2013 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 IOS_CHROME_COMMON_STRING_UTIL_H_
 #define IOS_CHROME_COMMON_STRING_UTIL_H_
 
 #import <CoreGraphics/CoreGraphics.h>
 #import <Foundation/Foundation.h>
 #include <string>
 
 // Parses a string with an embedded link inside, delineated by BEGIN_LINK and
 // END_LINK. Returns the string without the link delimiters. If |out_link_range|
 // is not null, then it is filled out with the range of the link in the returned
 // string.
 // If no link is found, then it returns |text| and sets |out_link_range| to
 // {NSNotFound, 0}.
 NSString* ParseStringWithLink(NSString* text, NSRange* out_link_range);
 
 // Utility method that returns an NSCharacterSet containing Unicode graphics
 // and drawing characters (but not including the Braille Patterns characters).
 NSCharacterSet* GraphicCharactersSet();
 
 // Cleans an NSString by collapsing whitespace and removing leading and trailing
 // spaces. If |removeGraphicChars| is true, unicode graphic characters will also
 // be removed from the string.
 NSString* CleanNSStringForDisplay(NSString* dirty, BOOL removeGraphicChars);
 
 // Cleans a std::string identically to CleanNSStringForDisplay()
-std::string CleanStringForDisplay(std::string dirty, BOOL removeGraphicChars);
+std::string CleanStringForDisplay(const std::string& dirty,
+                                  BOOL removeGraphicChars);
 
 // Find the longest leading substring of |string| that, when rendered with
 // |attributes|, will fit on a single line inside |targetWidth|. If |trailing|
 // is YES, then find the trailing (instead of leading) substring.
 NSString* SubstringOfWidth(NSString* string,
                            NSDictionary* attributes,
                            CGFloat targetWidth,
                            BOOL trailing);
 
 #endif  // IOS_CHROME_COMMON_STRING_UTIL_H_