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

This CL removes some unnecessary NULL-checks for Frame::selection().

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23532026

git-svn-id: svn://svn.chromium.org/blink/trunk@157097 bbb929c8-8fbe-4397-9dbb-9b2b20218538