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
Update FrameTree pipelines when appropriate

Instead of simply creating a new FrameTree when an iframe starts a
load, update the existing FrameTree's pipeline. This prevents the
FrameTree from accumulating many extra children.
Add FrameTree::add_child

This helper method simplifies a few lines.
Notify compositor when a FrameTree changes pipelines

When the constellation change a FrameTree pipeline, it should send a
notification to the compositor, so that it can negotiate buffers from
the correct render task. This commit also migrates away from the
generalized FrameTreeUpdateMsg to ones specific to the situation at
hand. That turned out to be an unnecessary abstraction.
Only send SetLayerRect messages for compositor-backed frames

Only send SetLayerRect when we know that a frame has a compositor
layer. If it doesn't, we wait until we send the new layer to the
compositor.