Eliminate SES guest frames and execute guest code in taming frame.

* In ES5 mode, use the taming frame instead of the guest frame for
  evaluating guest code.
* Don't create a guest frame.
* Rename ses-taming-frame.js to ses-single-frame.js.

This improves performance by loading fewer frames, especially when there
are multiple guests. It also makes way for being able to operate in the
presence of the 'Firefox 15 cross-frame freeze bug', since the objects
exposed to the guest are no longer from a different frame.

This also removes the possibility of implementing "CES" (guest frames
with unfrozen primordial objects); we will have to revert these changes
if that becomes needed).

Caveats:
* guestManager.iframe is now always the combined frame, and if clients
  depended on it being distinct they are now wrong. We should deprecate
  that access and give it a more clearly for-internal-testing-only name.
* mitigateGotchas is no longer excluded from the taming frame because
  that distinction does not exist.

@r5351

[kpreid2, 2013-04-12 01:00:07]

Notes:

- This review currently incorporates cleanup
https://codereview.appspot.com/8687044/ ; please review that one first so we can
get it out of the way as independent of this change.

- Suppressing mitigateGotchas may not be as important as we were thinking; all
of the major taming code is already loaded as part of the ses-taming-frame.js
concatenation, so is not subject to mitigateGotchas processing.

[ihab.awad, 2013-04-12 02:10:00]

Brilliant! LGTM!