Add WTF::String::emptyString16Bit

Source/wtf/text/StringImpl.h

Index: Source/wtf/text/StringImpl.h
diff --git a/Source/wtf/text/StringImpl.h b/Source/wtf/text/StringImpl.h
index 2c35a29413685f9f03114ef3759f841a3612a8e5..3df9d64ae4ebf8ebe29dc5110ea1ae349f2a2bdf 100644
--- a/Source/wtf/text/StringImpl.h
+++ b/Source/wtf/text/StringImpl.h
@@ -125,13 +125,13 @@ private:
     // Used to construct static strings, which have an special refCount that can never hit zero.
     // This means that the static string will never be destroyed, which is important because
     // static strings will be shared across threads & ref-counted in a non-threadsafe manner.
-    enum ConstructEmptyStringTag { ConstructEmptyString };
-    explicit StringImpl(ConstructEmptyStringTag)
+    enum ConstructEmptyStringTag { ConstructEmpty8BitString, ConstructEmpty16BitString };
+    explicit StringImpl(ConstructEmptyStringTag tag)
         : m_refCount(1)
         , m_length(0)
         , m_hash(0)
         , m_isAtomic(false)
-        , m_is8Bit(true)
+        , m_is8Bit(tag == ConstructEmpty8BitString)

[Mikhail, 2014/09/23 09:48:10]

think this is not enough, see below there is:
STRING_STATS_ADD_8BIT_STRING(m_length);

but it could be 

STRING_STATS_ADD_16BIT_STRING(m_length);

Also introducing an extra comparison ("tag == ConstructEmpty8BitString") is not
positive performance wise, WTF::String code is extremely hot.

IMO it's better to have a separate
enum ConstructEmpty16BitStringTag { ConstructEmpty16BitString };

and a separate constructor.

         , m_isStatic(true)
     {
         // Ensure that the hash is computed so that AtomicStringHash can call existingHash()
@@ -290,7 +290,9 @@ public:
         --m_refCount;
     }
 
+    enum Force16Bit { Force16BitEmpty };
     static StringImpl* empty();
+    static StringImpl* empty(Force16Bit);
 
     // FIXME: Does this really belong in StringImpl?
     template <typename T> static void copyChars(T* destination, const T* source, unsigned numCharacters)