We incorrectly mark a KeyedStoreIC miss as a monomorphic prototype
failure even though it's the first time a particular (string) key has
been seen.

BUG=358088
R=verwaest@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/219313002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20407 ce2b1a6d-e550-0410-aec6-3dcde31c8c00