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/uievent.rs

@@ -87,7 +87,7 @@ impl UIEvent {
 impl UIEventMethods for UIEvent {
     // https://w3c.github.io/uievents/#widl-UIEvent-view
     fn GetView(&self) -> Option<Root<Window>> {
-        self.view.get().map(Root::from_rooted)
+        self.view.get_rooted()
     }
 
     // https://w3c.github.io/uievents/#widl-UIEvent-detail