When following an accessor transition for an already existing accessor, don't load the last added descriptor but the same descriptor as we already found previously.

BUG=137689
TEST=test/mjsunit/regress/regress-crbug-137689.js

Review URL: https://chromiumcodereview.appspot.com/10808005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12115 ce2b1a6d-e550-0410-aec6-3dcde31c8c00