For top-level code the closure passed into context allocation methods
needs to be replaced with a sentinel to canonicalize is to the empty
function object.

R=bmeurer@chromium.org
TEST=mjsunit/regress/regress-4169
BUG=v8:4169
LOG=N

Review URL: https://codereview.chromium.org/1172013002

Cr-Commit-Position: refs/heads/master@{#28888}