This fixes a corner case when the instance prototype of a function is
changed while inobject slack tracking is still in progress. This caused
the intial map to be unrelated for functions with the same shared info
and hence the shared construct stub is no longer generic enough to work
for all those functions.

R=danno@chromium.org
BUG=chromium:157019
TEST=mjsunit/regress/regress-crbug-157019

Review URL: https://codereview.chromium.org/11293059

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12896 ce2b1a6d-e550-0410-aec6-3dcde31c8c00