Have LinkLoader::loadLink() take a Document reference in parameter

Have LinkLoader::loadLink() take a Document reference in parameter instead
of a pointer since it can never be NULL. This avoids having to convert
a reference into a pointer at call sites.

R=tkent
BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23833004

git-svn-id: svn://svn.chromium.org/blink/trunk@157072 bbb929c8-8fbe-4397-9dbb-9b2b20218538