Auto merge of #8477 - asajeffrey:opaque-domstring, r=asajeffrey

Opaque DOMString

This patch makes DOMString an opaque wrapper round String (currently it's a transparent wrapper).

The changes are:

* Replacing DOMString(foo) by DOMString::from(foo).
* Replacing foo.0 by String::from(foo).
* Adding functions clear, push_str and extend for in-place mutation of DOMStrings.
* Replacing DOMString by String in other threads (devtools, storage and filereader).
* Making DOMString implement !Send.
* Removing the pub attribute from the contents of DOMString.

This enables experimenting with other string representations in the DOM.

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components/script/dom/workerglobalscope.rs

@@ -299,7 +299,7 @@ impl WorkerGlobalScopeMethods for WorkerGlobalScope {
 impl WorkerGlobalScope {
     pub fn execute_script(&self, source: DOMString) {
         match self.runtime.evaluate_script(
-            self.reflector().get_jsobject(), source.0, self.worker_url.serialize(), 1) {
+            self.reflector().get_jsobject(), String::from(source), self.worker_url.serialize(), 1) {
             Ok(_) => (),
             Err(_) => {
                 // TODO: An error needs to be dispatched to the parent.