The GC prologue is called by the GC, but also by the heap snapshotter. The
RetainedObjectInfos are only needed by the heap snapshotter, so it's wasteful to
construct them always. (And it will be even more wasteful when Blink migrates to
the new GC APIs, since after that point it no longer knows about object groups.)

BUG=

Review URL: https://codereview.chromium.org/14471028
Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14439 ce2b1a6d-e550-0410-aec6-3dcde31c8c00