Refactor Context::Lookup so it is more obvious.  Change the comment in
contexts.h so it no longer indicates that it can return an arguments
object (it can't) and clean up the call sites that had leftover dead code.

BUG=
TEST=

Review URL: http://codereview.chromium.org/7782030

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9223 ce2b1a6d-e550-0410-aec6-3dcde31c8c00