In order to tailor certain security checks to the caller that is
attempting to load a particular piece of content, we need to be able to
attach an appropriate triggering principal to the corresponding requests.
Since most HTML content is loaded based on attribute values, this means
capturing the subject principal of the caller who sets those attributes,
which in turn means we need to make it available to `AfterSetAttr` hooks
on all relevant element types.
Since these are just interpreted comments, there's 0 impact on actual code.
This removes all lines that match /* vim: set(.*)tw=80: */ with S&R -- there are
a few others scattered around which will be removed manually in a second part.
This part of the bug was significantly complicated by the following major refactors:
https://bugzilla.mozilla.org/show_bug.cgi?id=1456435https://bugzilla.mozilla.org/show_bug.cgi?id=1459498
As best as I can tell, we just need to implement the explicitly enabled
flag on every instance of GetStyleSheetInfo, make sure
aIsExplicitlyEnabled is false in every situation except the one where
the disabled content attribute is removed from a link element, and
enable alternate stylesheets if this flag is set on them. So we take the
explicitly enabled flag as an input to PrepareSheet, and also add it to
LoadStyleLink and LoadInlineStyle. I also decided not to defer loading of
alternate stylesheets that have been explicitly enabled.
