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..d763219fa19d519cf6659652d5fa763ff94aa01d 100644
--- a/Source/wtf/text/StringImpl.h
+++ b/Source/wtf/text/StringImpl.h
@@ -141,6 +141,19 @@ private:
         hash();
     }
 
+    enum ConstructEmptyString16BitTag { ConstructEmptyString16Bit };
+    explicit StringImpl(ConstructEmptyString16BitTag)
+        : m_refCount(1)
+        , m_length(0)
+        , m_hash(0)
+        , m_isAtomic(false)
+        , m_is8Bit(false)
+        , m_isStatic(true)
+    {
+        STRING_STATS_ADD_16BIT_STRING(m_length);
+        hash();
+    }
+
     // FIXME: there has to be a less hacky way to do this.
     enum Force8Bit { Force8BitConstructor };
     StringImpl(unsigned length, Force8Bit)
@@ -291,6 +304,7 @@ public:
     }
 
     static StringImpl* empty();
+    static StringImpl* empty16Bit();
 
     // FIXME: Does this really belong in StringImpl?
     template <typename T> static void copyChars(T* destination, const T* source, unsigned numCharacters)