Merge StyleSheetCollection into ScopedStyleResolver.

Merge StyleSheetCollection into ScopedStyleResolver.

There's no reason to keep two identical lists of the sheets
and have this separate object. I also moved the updating
logic out of StyleResolver and into ScopedStyleResolver
which makes more sense. There's still some weirdness since
some global state still exists in the StyleResolver, but
that's something we can fix in future patches.

R=ojan@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/3c623f54af5118489abbeb7e803c94dec8dddd46

[ojan, 2015-01-14 02:30:52]

lgtm

[esprehn, 2015-01-14 02:37:07]

Committed patchset #1 (id:1) manually as
3c623f54af5118489abbeb7e803c94dec8dddd46 (presubmit successful).