Clearing this stub doesn't actually make sense, as it neither leaks
memory, nor prevents finding a new optimum stable state. As
long as strings receivers are observed, this is the best stub.
The moment we see a non-string receiver we'll go polymorphic
or generic depending on the case.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/605013003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24360 ce2b1a6d-e550-0410-aec6-3dcde31c8c00