Handle setters on the prototype chain efficiently by default.

Handle setters on the prototype chain efficiently by default.

Note that changes on the prototype chain should already be handled by
StubCompiler::CheckPrototypes, if not, it is a bug there and not in
LookupForWrite. Furthermore, neither test262 nor WebKit layout tests hint at a
compability issue, so it should be safe to remove a redundant check.

The good point about this CL: It improves Box2D's performance by 22%.

Review URL: https://chromiumcodereview.appspot.com/10695197

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12079 ce2b1a6d-e550-0410-aec6-3dcde31c8c00