Don't attempt to transition JSProxy to an observed map / Object.getNotifier() must SetObserved

I found this working on 

https://codereview.chromium.org/19541010/

The main problem is that if you called Object.getNotifier(obj) on an object, %SetObserved(object) would never get called on it, and thus it would be unobservable (new test added for this).

Additionally, Runtime::SetObserved was asserting obj->IsJSObject() which would fail if called on a proxy.

It just happens that our existing test always called getNotifier() before Object.observe on proxies, and thus we never previously attempted to transition the map of a proxy.

Both issues are now fixed and properly tested.

R=rossberg@chromium.org

Committed: http://code.google.com/p/v8/source/detail?r=16074

[rossberg, 2013-08-06 13:47:29]

LGTM, landing.

[rossberg, 2013-08-06 13:49:16]

Committed patchset #1 manually as r16074 (presubmit successful).