Make DomRoot<T> a type alias of a more general Root<T> type

This Root<T> relies on the concept of a stable trace object, which doesn't
move for the whole lifetime of the T value. For safety reasons, T must be 'static.

The only implementor of StableTraceObject for now is Dom<T>.

components/script/dom/bindings/reflector.rs

@@ -72,7 +72,7 @@ impl Reflector {
 }
 
 /// A trait to provide access to the `Reflector` for a DOM object.
-pub trait DomObject {
+pub trait DomObject: 'static {
     /// Returns the receiver's reflector.
     fn reflector(&self) -> &Reflector;