Avoid adjusting live bytes in JSObject::MigrateFastToFast() if the size delta is zero.

BUG=chromium:388880
LOG=N
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/333903003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22031 ce2b1a6d-e550-0410-aec6-3dcde31c8c00