Trimming the SecurityPolicy DOM interface

As discussed at the face-to-face meeting, I've trimmed the
SecurityPolicy DOM interface to just the first four attributes:

https://dvcs.w3.org/hg/content-security-policy/rev/f338192860c5

At the meeting, we discussed that these attribute have strong use
cases, but we couldn't think of any strong use cases for the remaining
DOM interfaces.

If folks come up with strong use cases, we should consider adding back
the removed interfaces (or adding new interfaces that better address
those use cases).

Note: At the face-to-face, we discussed making some of these attribute
writable in some circumstances, but I haven't made that change yet
because it probably deserves more discussion.

Adam