One of the fast cases in JSObject::MigrateFastToFast() should not be taken if the number of fields did not change.

BUG=chromium:390918
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/363073002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22174 ce2b1a6d-e550-0410-aec6-3dcde31c8c00