Add [TreatReturnedNullStringAs] support for ByteString.

Source/core/testing/TypeConversions.idl

Index: Source/core/testing/TypeConversions.idl
diff --git a/Source/core/testing/TypeConversions.idl b/Source/core/testing/TypeConversions.idl
index 3144c0d8290ca9ce3606674c0d1682ded33ac41e..42474aad8fbbf45e8e54cacd3668e9b6c0e97bad 100644
--- a/Source/core/testing/TypeConversions.idl
+++ b/Source/core/testing/TypeConversions.idl
@@ -47,6 +47,10 @@
     [EnforceRange, ImplementedAs=testUnsignedShort] attribute unsigned short testEnforceRangeUnsignedShort;
 
     attribute ByteString testByteString;
+    [ImplementedAs=testByteString, TreatReturnedNullStringAs=Null] attribute ByteString testByteStringTreatReturnedNullStringAsNullAttribute;
+    [ImplementedAs=testByteString, TreatReturnedNullStringAs=Undefined] attribute ByteString testByteStringTreatReturnedNullStringAsUndefinedAttribute;
+    [ImplementedAs=testByteString, TreatReturnedNullStringAs=Null] ByteString getTestByteStringTreatReturnedNullStringAsNullMethod();
+    [ImplementedAs=testByteString, TreatReturnedNullStringAs=Undefined] ByteString getTestByteStringTreatReturnedNullStringAsUndefinedMethod();
     void setTestByteString(ByteString byteString);
     [ImplementedAs=setTestByteString] void setTestByteStringDefaultNull(optional ByteString byteString = null);
 };