Consider / prototype using platform specific direct compositing APIs.

[glennw]

We'd like to experiment with using DirectComposite and CoreAnimation, to see what benefits there would be.

We think that a good way to prototype an initial implementation would be to implement DirectComposite for Document layers. This should be:

• A relatively straightforward implementation.
• Significant wins (e.g. not building the main content when there are loading spinners in the tab bar).
• Allow us to get a basic implementation running while working out the details of using it for content.

[glennw]

cc @kvark @jrmuizel @nical