Use IDL argument default values in modules/encoding/

Source/modules/encoding/TextDecoder.cpp

Index: Source/modules/encoding/TextDecoder.cpp
diff --git a/Source/modules/encoding/TextDecoder.cpp b/Source/modules/encoding/TextDecoder.cpp
index 59e5e2603d55acf79e50a07ef99bdde965994d4b..2de005cffaa1dcc2a297ea2e57727f0a8cc0a0e2 100644
--- a/Source/modules/encoding/TextDecoder.cpp
+++ b/Source/modules/encoding/TextDecoder.cpp
@@ -41,13 +41,11 @@ namespace WebCore {
 
 TextDecoder* TextDecoder::create(const String& label, const Dictionary& options, ExceptionState& exceptionState)
 {
-    const String& encodingLabel = label.isNull() ? String("utf-8") : label;
-
-    WTF::TextEncoding encoding(encodingLabel);
+    WTF::TextEncoding encoding(label);
     // The replacement encoding is not valid, but the Encoding API also
     // rejects aliases of the replacement encoding.
     if (!encoding.isValid() || !strcasecmp(encoding.name(), "replacement")) {
-        exceptionState.throwTypeError("The encoding label provided ('" + encodingLabel + "') is invalid.");
+        exceptionState.throwTypeError("The encoding label provided ('" + label + "') is invalid.");
         return 0;
     }