The three tests that are marked as failures are because they use offsetWidth as a dummy to force a layout flush. Now that these exist, they expose an unrelated bug with reflow.
Implement getComputedStyle
It's not quite done but can probably be reviewed anyway.
I still need to finish up a few of the ToCss impls, I just got lazy and wanted to make sure things worked.
The computation of the used values is definitely not right, I'm going to investigate that.
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6798)
<!-- Reviewable:end -->
Passes some but not all WPT tests. One failure is caused by an issue in codegen for the `DOMPointInit` dictionary, the others by outdated tests: Gecko implements an old version of the spec that overloaded the `DOMPoint` constructor to optionally take an object as the first argument, and made `DOMPointReadOnly` non-constructible.
