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

ios/chrome/common/string_util.mm

 // 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.
 
 #include "ios/chrome/common/string_util.h"
 
 #import <UIKit/UIKit.h>
 
 #include "base/logging.h"
 #include "base/mac/scoped_block.h"
@@ skipping 98 matching lines @@
   ArrayFilterProcedure filter = ^(id object, NSUInteger index, BOOL* stop) {
     return [object isEqualToString:@""];
   };
   [spaceSeparatedCompoments
       removeObjectsAtIndexes:[spaceSeparatedCompoments
                                  indexesOfObjectsPassingTest:filter]];
   cleanString = [spaceSeparatedCompoments componentsJoinedByString:@" "];
   return cleanString;
 }
 
-std::string CleanStringForDisplay(std::string dirty, BOOL removeGraphicChars) {
+std::string CleanStringForDisplay(const std::string& dirty,
+                                  BOOL removeGraphicChars) {
   return base::SysNSStringToUTF8(CleanNSStringForDisplay(
       base::SysUTF8ToNSString(dirty), removeGraphicChars));
 }
 
 NSString* SubstringOfWidth(NSString* string,
                            NSDictionary* attributes,
                            CGFloat targetWidth,
                            BOOL trailing) {
   if (![string length])
     return nil;
@@ skipping 38 matching lines @@
       else {
         substring = getSubstring.get()(characters - increment);
         break;  // Growing the string, found the right size.
       }
     }
     prevWidth = thisWidth;
   } while (characters > 0 && characters < [string length]);
 
   return substring;
 }