contain: strict hacky patches

[Julien Chaffraix]

Hi everyone,

last week, there was some informal hackathon in Sydney and I worked on
implementing contain: strict. I implemented several strictly contained
restrictions [1] along with the parsing / getComputedStyle, not the
performance optimizations (which will require some rearchitecturing to
reap the benefits).

As I didn't volunteer to take over the feature, this may interest some
people! The patches need more clean-ups but they should provide a good
start.

Cheers,
Julien

[1] http://dev.w3.org/csswg/css-containment/#strictly-contained