Fix memory leak in the debugger.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/297373002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21479 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Make let variables fresh in each iteration of a for-loop.

BUG=v8:2198
LOG=N
TEST=mjsunit/harmony/block-for
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/292743009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21480 ce2b1a6d-e550-0410-aec6-3dcde31c8c00