Have DOMWindow deal with references instead of pointers when possible

Have DOMWindow deal with references instead of pointers when possible. This
makes the code look safer and gets rid of unnecessary null checks.

This CL also updates the generated bindings so that they allow for the
implementation to return references instead of pointers in more cases.

R=adamk@chromium.org, nbarth@chromium.org, adamk

Review URL: https://codereview.chromium.org/176763009

git-svn-id: svn://svn.chromium.org/blink/trunk@168422 bbb929c8-8fbe-4397-9dbb-9b2b20218538