Support nullable strings in dictionaries.

We copy the string in all cases, so there's no need to handle the isMember
case differently.

src/components/script/dom/bindings/codegen/CodegenRust.py

@@ -702,13 +702,6 @@ def getJSToNativeConversionTemplate(type, descriptorProvider, failureCode=None,
 
             return handleDefault(conversionCode, default)
 
-        if isMember:
-            # We have to make a copy, because our jsval may well not
-            # live as long as our string needs to.
-            declType = CGGeneric("DOMString")
-            return ("%s\n" % getConversionCode(),
-                    declType, None, isOptional, None)
-
         declType = "DOMString"
         initialValue = None
         if type.nullable():