Unravel kHeapObjectTagSize from the stub cache.

The stub cache used kHeapObjectTagSize to scale indices, but there
doesn't appear to be a direct need for this. Instead, the stub cache has
its own kCacheIndexShift quantity.

BUG=
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/401613003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22466 ce2b1a6d-e550-0410-aec6-3dcde31c8c00