Instead of NULL in the previous field of function contexts, put the previous
context.  This saves the indirection of fetching the previous through the
context's closure.

R=ager@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/7134042

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8238 ce2b1a6d-e550-0410-aec6-3dcde31c8c00