Fix a bug in Lithium environment iteration.

Fix a bug in Lithium environment iteration.

The Advance() function of the class responsible for iterating
environment uses didn't always advance as far as it could (relying on
the HasNext predicate to finish advancing).  This is brittle.

The HasNext predicate also didn't advance as far as it could when it
was at the end of an environment level.  This is a bug.

R=jkummerow@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/6993023

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8181 ce2b1a6d-e550-0410-aec6-3dcde31c8c00