Before: every context cached the nearest enclosing function context.  This
assumed that for nested contexts (i.e., with and catch contexts) the
enclosing function had a materialized link in the context chain.

Now: when necessary, we loop up the context chain to find such a context.
This enables catch contexts without forcing the enclosing function to
allocate its own context.

R=ager@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/7230047

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8452 ce2b1a6d-e550-0410-aec6-3dcde31c8c00