Have HashMap<KeyType, AtomicString>::get() return a const reference

Source/wtf/text/AtomicStringHash.h

 /*
  * Copyright (C) 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 32 matching lines @@
         static bool equal(const AtomicString& a, const AtomicString& b)
         {
             return a == b;
         }
 
         static const bool safeToCompareToEmptyOrDeleted = false;
     };
 
     // AtomicStringHash is the default hash for AtomicString
     template<> struct HashTraits<WTF::AtomicString> : GenericHashTraits<WTF::AtomicString> {
+        // Unlike other types, we can return a const reference for AtomicString's empty value (nullAtom).
+        typedef const WTF::AtomicString& PeekType;
+
+        static const WTF::AtomicString& emptyValue() { return nullAtom; }
+        static PeekType peek(const WTF::AtomicString& value) { return value; }
+
         static const bool emptyValueIsZero = true;
         static void constructDeletedValue(WTF::AtomicString& slot) { new (NotNull, &slot) WTF::AtomicString(HashTableDeletedValue); }
         static bool isDeletedValue(const WTF::AtomicString& slot) { return slot.isHashTableDeletedValue(); }
     };
 
 }
 
 using WTF::AtomicStringHash;
 
 #endif