Fix issue 186:...

Fix issue 186:

    http://code.google.com/p/v8/issues/detail?id=186

Create a new instance type for context extension objects.  Use it to
not use the __proto__ accessor for context extension objects.

Committed: http://code.google.com/p/v8/source/detail?r=1072

[Kevin Millikin (Chromium), 2009-01-14 12:10:09]

Annoying to have to have such a special case, but LGTM.