In generated code, when walking the context to inspect context extension
objects, use the previous context link directly.

R=ager@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/7044081

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8248 ce2b1a6d-e550-0410-aec6-3dcde31c8c00