The main missing features are rootMargin, proper nested browsing
context support and content clip/clip-path support.
This makes images appear on some sites, such as YouTube and
howstuffworks.com.
Note there are a couple of type differences between the spec and the IDL
file added in this commit. For example, we will need to support a type
of Variant to handle spec types such as "(double or sequence<double>)".
But for now, this allows web pages to construct an IntersectionObserver
with any valid type.
