Remove the document member on FontLoader. The FontLoader is already a ContextLifecycleObserver and therefore has access to the document.

R=abarth@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/23618041

git-svn-id: svn://svn.chromium.org/blink/trunk@157568 bbb929c8-8fbe-4397-9dbb-9b2b20218538