Add WebPublicSuffixList interface.

The public suffix list specifies the set of top-level domains.  For example, it tells us that for google.co.uk,
co.uk is the top-level domain, not just uk.

This will be used shortly to prevent setting document.domain to top-level domains.

BUG=307407

Review URL: https://codereview.chromium.org/67423002

git-svn-id: svn://svn.chromium.org/blink/trunk@161988 bbb929c8-8fbe-4397-9dbb-9b2b20218538