Node since it is used by text nodes and document nodes as well.
Specs: Make it possible for an element to set what layout manager
class it uses.
Specs: Expose the root layout manager as mutable state.
Specs: Update the runloop to have more detail.
Specs: Add support for transitions, by making the resolver for a value
know when it has been called for the first time, and giving it access
to the previous cached value, and letting it store state on the render
node.
Other sundry changes.
Review URL: https://codereview.chromium.org/835973003