Simplify synthetic roots creation in heap snapshot

The synthetic roots exist in every snapshot and there are no actual objects corresponding to them (there was attempt to create some virtual objects to reuse existing snapshot builder but it just complicated the things).

Previously synthetic root wouldn't have been added if it was empty. Current implementation always adds all synthetic roots in the snapshot no matter if they are empty or not.

BUG=None
LOG=N
R=alph@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/590833002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24124 ce2b1a6d-e550-0410-aec6-3dcde31c8c00