Fix missing access check in Runtime_SetPrototype.

R=rossberg@chromium.org
BUG=chromium:354123
TEST=cctest/test-api/Regress354123
LOG=y

Review URL: https://codereview.chromium.org/205033011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20138 ce2b1a6d-e550-0410-aec6-3dcde31c8c00