Tell the GC prologues to construct RetainedObjectInfos only when needed.

Tell the GC prologues to construct RetainedObjectInfos only when needed.

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=

Committed: https://code.google.com/p/v8/source/detail?r=14439

[marja, 2013-04-25 09:36:22]

hpayer, pls review.

mstarzinger ACKed this approach, but we didn't discuss naming the flag.

[Hannes Payer (out of office), 2013-04-25 11:47:11]

LGTM

[Hannes Payer (out of office), 2013-04-25 12:08:17]

Committed patchset #1 manually as r14439 (presubmit successful).