Ported commits: r8126 (6461bae)

Original commit message:
- Introduce a class JSReceiver, that is a common superclass of JSObject and
JSProxy. Use JSReceiver where appropriate (probably lots of places that we
still have to migrate, but we will find those later with proxy test suite).
- Move appropriate methods to JSReceiver class (SetProperty,
GetPropertyAttribute, Get/SetPrototype, Lookup, and so on).
- Introduce new JSFunctionProxy subclass of JSProxy. Currently only a stub.
- Overhaul enum InstanceType:
* Introduce FIRST/LAST_SPEC_OBJECT_TYPE that ranges over all types that
represent JS objects, and use that consistently to check language types.
* Rename FIRST/LAST_JS_OBJECT_TYPE and FIRST/LAST_FUNCTION_CLASS_TYPE
to FIRST/LAST_[NON]CALLABLE_SPEC_OBJECT_TYPE for clarity.
* Eliminate the overlap over JS_REGEXP_TYPE.
* Also replace FIRST_JS_OBJECT with FIRST_JS_RECEIVER, but only use it where
we exclusively talk about the internal representation type.
* Insert JS_PROXY and JS_FUNCTION_PROXY in the appropriate places.
- Fix all checks concerning classification, especially for functions, to
use the CALLABLE_SPEC_OBJECT range (that includes funciton proxies).
- Handle proxies in SetProperty (that was the easiest part :) ).
- A few simple test cases.

BUG=
TEST=

Review URL: http://codereview.chromium.org/7024041
Patch from Paul Lind <plind44@gmail.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8175 ce2b1a6d-e550-0410-aec6-3dcde31c8c00