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

Cr-Commit-Position: refs/heads/master@{#24902}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24902 ce2b1a6d-e550-0410-aec6-3dcde31c8c00