Delete StyleSheetList and support code.

We don't need this since we don't expose the list of sheets from Document
or ShadowRoot in Sky. After removing this code I also simplified the
system and deleted StyleSheetCandidate and DocumentStyleSheetCollector
which don't really do anything anymore.

Even if we do want to add back a list of sheets later it won't need code
because we don't have a concept of a remote sheet like <link rel> did,
and we don't have to deal with non-CSS sheets.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/803673003