Refactor away duplication of get_rooted functionality

Refactor .get().map(Root::from_rooted)
and .get().map(|foo| foo.root())
to .get_rooted() on MutNullableHeap objects.

First part done mechanically with the following comand:
sed -i s/"get().map(Root::from_rooted)"/"get_rooted()"/g *.rs

Second part done manually after finding them with
git grep ".get().map("

Fixes 7929.

components/script/dom/xmlhttprequest.rs

@@ -724,7 +724,7 @@ impl XMLHttpRequestMethods for XMLHttpRequest {
 
     // https://xhr.spec.whatwg.org/#the-responsexml-attribute
     fn GetResponseXML(&self) -> Option<Root<Document>> {
-        self.response_xml.get().map(Root::from_rooted)
+        self.response_xml.get_rooted()
     }
 }