Change StyleResolver::document() to be private

Moved StyleResolver::document() into the private section and changed all callers to get the document from other sources.

Review URL: https://codereview.chromium.org/579253002

git-svn-id: svn://svn.chromium.org/blink/trunk@183333 bbb929c8-8fbe-4397-9dbb-9b2b20218538