Move PageRuleCollector off of StyleResolverState

PageRuleCollector doesn't need most of StyleResolverState,
so moved to ElementResolveContext instead.
I could even have moved it just to have a m_rootElement pointer
but decided that was extreme.  PageRuleCollector looks
possibly half-implemented, so it may have more needs yet.

BUG=259202

Review URL: https://chromiumcodereview.appspot.com/18462005

git-svn-id: svn://svn.chromium.org/blink/trunk@154169 bbb929c8-8fbe-4397-9dbb-9b2b20218538