Remove CSSOM mutability in StyleRule.

The StyleRule classes supported copy() operations and mutable
property sets so that we could do copy-on-write and mutate them
using the CSSOM. Sky doesn't have a CSSOM like this though,
so we can remove all this code and make the StyleRule classes
effectively immutable after construction (the parser does mutate some
rules though).

I also removed some other dead code in the StyleRule hierarchy,
like some left over mutation methods that the CSSOM used to use.

R=eseidel@chromium.org, ojan@chromium.org

Review URL: https://codereview.chromium.org/758573005