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

ios/chrome/common/string_util.mm

Index: ios/chrome/common/string_util.mm
diff --git a/ios/chrome/common/string_util.mm b/ios/chrome/common/string_util.mm
index d8fca0e71a6266a9389f853dc85c7f97beb9509e..395f66035276d4a244542b05aaa3a9063bc4469f 100644
--- a/ios/chrome/common/string_util.mm
+++ b/ios/chrome/common/string_util.mm
@@ -116,7 +116,8 @@ NSString* CleanNSStringForDisplay(NSString* dirty, BOOL removeGraphicChars) {
   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));
 }