Per the CSP specification, content injected by extensions is meant to
be exempt from page CSP. This patch takes care of the most common case
of content injected by extension content scripts, which always have
expanded principals which inherit from the page principal.
To make this easier, de-virtualize BasePrincipal::Kind(), using CTOR
initializers instead.
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.
