Remove outdated comment now that document() returns a reference.

The comment refers to code removed in r157023 which was an early return
when document() is null (which itself no longer occurs after r156581).

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23464048

git-svn-id: svn://svn.chromium.org/blink/trunk@157494 bbb929c8-8fbe-4397-9dbb-9b2b20218538