Have CSSParserContext take a Document reference in argument

Have CSSParserContext take a Document reference in argument instead of a
pointer as it can never be NULL. This avoids having to convert a reference
into a pointer at the call sites.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23516004

git-svn-id: svn://svn.chromium.org/blink/trunk@157071 bbb929c8-8fbe-4397-9dbb-9b2b20218538