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/bindings/codegen/CodegenRust.py

@@ -825,7 +825,7 @@ def getJSToNativeConversionInfo(type, descriptorProvider, failureCode=None,
             default = "None"
         else:
             assert defaultValue.type.tag() == IDLType.Tags.domstring
-            default = 'DOMString("%s".to_owned())' % defaultValue.value
+            default = 'DOMString::from("%s")' % defaultValue.value
             if type.nullable():
                 default = "Some(%s)" % default