There are other places we want to convert font-size into it's computed
form than just when we are loading the font (e.g. computing keyframes).
Gains us 36 WPT passes as we now correctly clamp negative calc values.
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.
