Outline ContentSecurityPolicy::document()

This allows the Document.h include to be removed from
ContentSecurityPolicy.h, which reduces recompilation when
touching Document.h from 1156 to 1101 steps locally.

Since document() was only used internally, make it private.

Review URL: https://codereview.chromium.org/348743006

git-svn-id: svn://svn.chromium.org/blink/trunk@176856 bbb929c8-8fbe-4397-9dbb-9b2b20218538