Merge pull request #1 from tkuehn/iframes

add rect to FrameTree on constellation side
This implements the DOM tree accessors that return a HTMLCollection
Fix some implementations for DOM tree accessors
Factor out CSS length to Au conversion
Simplify HTMLCollection predicates
Fill out various proxy binding traps as part of making setters work. Add named getter and setter and indexed getter support, as well as proxy object expandos. Fixes #660.
Implement clear
Update submodules
Place float children of floats correctly in the flow tree.
Fix the handling of flow_contexts in floats.

The num_floats was hard-coded to 1 and didn't take into account the
children. Also the float context was not being threaded through the children
properly.
placeholders for FrameRectMsg from Layout to Constellation
add rect to FrameTree
added CompositorLayer::from_frame_tree