Remove CSSOM mutability in StyleRule.

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

Committed: https://chromium.googlesource.com/external/mojo/+/2d17d1e99a5581d87023e2df62745b59f6558b32

[esprehn, 2014-12-03 23:00:50]

esprehn@chromium.org changed reviewers:
+ eseidel@chromium.org, ojan@chromium.org

[eseidel, 2014-12-03 23:02:14]

lgtm

You really don't write much in your descriptions do you...

[ojan, 2014-12-03 23:03:48]

lgtm

[esprehn, 2014-12-03 23:54:39]

Committed patchset #1 (id:1) manually as
2d17d1e99a5581d87023e2df62745b59f6558b32 (presubmit successful).