Make all references from root shortcuts (except one to GC roots)

Eventually we are going to nuke this shortcuts from root entry altogether. This fix makes (GC roots) retained size equal to the snapshot's total size.

BUG=chromium:414190
LOG=N
R=loislo@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/681743002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24902 ce2b1a6d-e550-0410-aec6-3dcde31c8c00