Cleanup Page::remove.
Remove comment that hasn't been true since Document became a Node.

The panic was removed in 99a36cbeb6077976861d94b7af16e9e57994a14d.
Cleanup PageIterator::next.
auto merge of #4315 : Ms2ger/servo/page, r=jdm
ports/cef: Implement accelerated compositing for the CEF port.
auto merge of #4193 : pcwalton/servo/cef-redux, r=pcwalton,metajack

@glennw, is it possible to rebase your stuff on top of this? Sorry for the mess.

r? @metajack
layout: Implement `text-transform`.

The Unicode awareness of `text-transform` is implemented as well as
possible given the Rust standard library's Unicode support. In
particular, the notion of an alphabetic character is used instead of a
letter.

Gecko has a subclass of text run to handle text transforms, but I
implemented this in a simpler way.
Address review comments