Don't clear the string stub at gc time in KeyedLoadIC.

Don't clear the string stub at gc time in KeyedLoadIC.

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