Refactoring: Frame::editor() should return a reference.

This CL removes some unnecessary NULL checks.
Inspired by http://trac.webkit.org/changeset/150140 by andersca.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23518002

git-svn-id: svn://svn.chromium.org/blink/trunk@156760 bbb929c8-8fbe-4397-9dbb-9b2b20218538