http://www.robohornet.org gives a score of 101.36 on master,
and 102.68 with this PR. The latter is slightly better,
but probably within noise level.
So it looks like this PR does not affect DOM performance.
This is expected since `Box::new` is defined as:
```rust
impl<T> Box<T> {
#[inline(always)]
pub fn new(x: T) -> Box<T> {
box x
}
}
```
With inlining, it should compile to the same as box syntax.
In a later PR, DomRoot<T> will become a type alias of Root<Dom<T>>,
where Root<T> will be able to handle all the things that need to be
rooted that have a stable traceable address that doesn't move for the
whole lifetime of the root. Stay tuned.
Consistently use the name 'pipeline_id' to refer to a function that
returns an (optional) PipelineId.
This was prompted by discovering both fn pipeline and fn pipeline_id
doing the same job in htmliframeelement.rs.
Note that there is fn pipeline in components/compositing/compositor.rs,
but that actually returns an Option<&CompositionPipeline>, not any kind
of PipelineId.
