Internalize string keys in Keyed{Store,Load}IC.

R=jkummerow@chromium.org
BUG=v8:3144
LOG=N

Review URL: https://codereview.chromium.org/162983003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19345 ce2b1a6d-e550-0410-aec6-3dcde31c8c00