Add WTF::String::emptyString16Bit

Source/wtf/text/WTFString.cpp

Index: Source/wtf/text/WTFString.cpp
diff --git a/Source/wtf/text/WTFString.cpp b/Source/wtf/text/WTFString.cpp
index d72969f19a6a82d4f4f9e513510ec2b01ad2749f..4059412a0d46cbad2741b7381cccfd498b6dd524 100644
--- a/Source/wtf/text/WTFString.cpp
+++ b/Source/wtf/text/WTFString.cpp
@@ -887,7 +887,7 @@ String String::make8BitFrom16BitSource(const UChar* source, size_t length)
 String String::make16BitFrom8BitSource(const LChar* source, size_t length)
 {
     if (!length)
-        return emptyString();
+        return emptyString(Force16BitEmpty);
 
     UChar* destination;
     String result = String::createUninitialized(length, destination);
@@ -1226,6 +1226,12 @@ const String& emptyString()
     return emptyString;
 }
 
+const String& emptyString(Force16Bit)
+{
+    DEFINE_STATIC_LOCAL(String, emptyString, (StringImpl::empty(StringImpl::Force16BitEmpty)));
+    return emptyString;
+}
+
 } // namespace WTF
 
 #ifndef NDEBUG