This implements the basic interface, classes and functions for
OffscreenCanvas. Many are still stubbed out and have many FIXMEs in
them, but it is a basic skeleton.
This is not really a context, but more of a set of parameters for
creating a Parser. So, treat it as such: Rename it to ParsingParams,
and store its values and methods directly in the Parser instead of
keeping the ParsingContext around.
This has a nice side-effect of not including DOM/Document.h everywhere
that needs a Parser.
