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..f2c9613a4c77a9f68ee47b56fcf0f5d2cba3b31d 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(false);
 
     UChar* destination;
     String result = String::createUninitialized(length, destination);
@@ -1220,10 +1220,11 @@ float charactersToFloat(const UChar* data, size_t length, size_t& parsedLength)
     return static_cast<float>(toDoubleType<UChar, AllowTrailingJunk>(data, length, 0, parsedLength));
 }
 
-const String& emptyString()
+const String& emptyString(bool is8Bit)
 {
-    DEFINE_STATIC_LOCAL(String, emptyString, (StringImpl::empty()));
-    return emptyString;
+    DEFINE_STATIC_LOCAL(String, empty8BitString, (StringImpl::empty()));
+    DEFINE_STATIC_LOCAL(String, empty16BitString, (StringImpl::empty(false)));
+    return is8Bit ? empty8BitString : empty16BitString;

[Mikhail, 2014/09/22 10:26:18]

I'm afraid this might affect the performance, what about

const String& emptyString() {}

and

enum Force16Bit { Force16Bit };
const String& emptyString(Force16Bit) {}

 }
 
 } // namespace WTF