Refactoring: Reduces the Document dependency on StyleSheet collection subclasses.

This is a cleanup that gets rid of some Document dependency.
In future change, the StyleEngine, not the Document, will lookup the appropriate
StyleResolver instance.

BUG=319151
R=tasak@google.com, esprehn, tasak

Review URL: https://codereview.chromium.org/73733003

git-svn-id: svn://svn.chromium.org/blink/trunk@162291 bbb929c8-8fbe-4397-9dbb-9b2b20218538