Bail out of CSP digest creation if no CSP validation is to be done

This shows up as the main CPU hog on the profile of a page containing
many large stylesheets whose contents are cached.

R=jww@chromium.org

Review URL: https://codereview.chromium.org/193953002

git-svn-id: svn://svn.chromium.org/blink/trunk@168896 bbb929c8-8fbe-4397-9dbb-9b2b20218538