Do not call setters of read-only accessors.

BUG=
TEST=mjsunit/readonly-accessor
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/271433002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21158 ce2b1a6d-e550-0410-aec6-3dcde31c8c00