Make DOMString a newtype around String, rather than a typedef.

This should make it somewhat easier to experiment with alternative
representations in the future. To reduce churn, this commit leaves the String
field public, though.

Also, this will allow us to use the default String type to represent the IDL
USVString type, which explicitly forbids unpaired surrogates, ans as such is
a better match to the Rust String type.

components/util/mem.rs

@@ -35,7 +35,7 @@ use std::mem::{size_of, transmute};
 use std::rc::Rc;
 use std::result::Result;
 use std::sync::Arc;
-use str::LengthOrPercentageOrAuto;
+use str::{DOMString, LengthOrPercentageOrAuto};
 use string_cache::atom::Atom;
 use string_cache::namespace::Namespace;
 use url;
@@ -101,6 +101,12 @@ impl HeapSizeOf for String {
     }
 }
 
+impl HeapSizeOf for DOMString {
+    fn heap_size_of_children(&self) -> usize {
+        self.0.heap_size_of_children()
+    }
+}
+
 impl<T: HeapSizeOf> HeapSizeOf for Option<T> {
     fn heap_size_of_children(&self) -> usize {
         match *self {