Link function contexts directly to the previous context.

Link function contexts directly to the previous context.

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