Temporarily disable DoStoreContextSlot in x64 lithium codegen to diagnose non-reproducible buildbot error.  Fix presubmit error.
Review URL: http://codereview.chromium.org/6572002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6915 ce2b1a6d-e550-0410-aec6-3dcde31c8c00