The spec is unclear about when exactly we should parse the style sheet.
Previously we'd do so before calling this algorithm, which was
error-prone, as seen by the bug fixed by the previous commit. The spec
for step 1 of "create a CSS style sheet" says:
1. Create a new CSS style sheet object and set its properties as
specified.
The definitions linked are UA-defined enough that it seems reasonable to
put the parsing here. That simplifies the user code a little and makes
it harder to mess up. It does raise the question of what to do if
parsing fails. I've matched our previous behaviour by just logging and
returning in that case.
While I'm modifying this method, I've also converted the bool params to
enums so they're a little clearer to read.
This is ad-hoc, and the spec doesn't seem to tell us what to actually do
here. Without this, following the spec steps for loading relative
`@import` URLs from a `<style>` tag always fails, because that uses the
parent style sheet's location as the base URL.
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.
