Remove AtomicStringImpl.

Source/wtf/text/AtomicString.cpp

Index: Source/wtf/text/AtomicString.cpp
diff --git a/Source/wtf/text/AtomicString.cpp b/Source/wtf/text/AtomicString.cpp
index 685016e3b461b7060c35cc33aabea2306a9bda24..a949c00ed07667172c2b162d3b80bae3294fb40c 100644
--- a/Source/wtf/text/AtomicString.cpp
+++ b/Source/wtf/text/AtomicString.cpp
@@ -404,13 +404,13 @@ static inline HashSet<StringImpl*>::iterator findString(const StringImpl* string
     return stringTable().find<HashAndCharactersTranslator<CharacterType> >(buffer);
 }
 
-AtomicStringImpl* AtomicString::find(const StringImpl* stringImpl)
+StringImpl* AtomicString::find(const StringImpl* stringImpl)
 {
     ASSERT(stringImpl);
     ASSERT(stringImpl->existingHash());
 
     if (!stringImpl->length())
-        return static_cast<AtomicStringImpl*>(StringImpl::empty());
+        return StringImpl::empty();
 
     HashSet<StringImpl*>::iterator iterator;
     if (stringImpl->is8Bit())
@@ -419,7 +419,7 @@ AtomicStringImpl* AtomicString::find(const StringImpl* stringImpl)
         iterator = findString<UChar>(stringImpl);
     if (iterator == stringTable().end())
         return 0;
-    return static_cast<AtomicStringImpl*>(*iterator);
+    return *iterator;
 }
 
 void AtomicString::remove(StringImpl* r)