Auto merge of #6660 - nox:children-changed, r=jdm

Introduce VirtualMethods::children_changed()

This virtual method mimics the behaviour of mutation observers and make it more viable than the older child_inserted(), which didn't cover removed nodes and was called as many times as there were inserted nodes.

A few other shortcomings where remove_child() was called directly instead of Node::remove() were also fixed while at it.

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components/script/dom/bindings/trace.rs

@@ -70,6 +70,7 @@ use std::collections::hash_state::HashState;
 use std::ffi::CString;
 use std::hash::{Hash, Hasher};
 use std::intrinsics::return_address;
+use std::mem;
 use std::ops::{Deref, DerefMut};
 use std::rc::Rc;
 use std::sync::Arc;
@@ -482,6 +483,13 @@ impl<T: JSTraceable + Reflectable> RootedVec<T> {
     }
 }
 
+impl<T: JSTraceable + Reflectable> RootedVec<JS<T>> {
+    /// Obtain a safe slice of references that can't outlive that RootedVec.
+    pub fn r(&self) -> &[&T] {
+        unsafe { mem::transmute(&*self.v) }
+    }
+}
+
 impl<T: JSTraceable + Reflectable> Drop for RootedVec<T> {
     fn drop(&mut self) {
         RootedTraceableSet::remove(self);