MarkCompactCollector::Prepare is too early in the rare case that we don't have a
marking deque yet. EnterFinalPause could then trigger
Heap::RegisterExternallyReferencedObject which rightfully fails because MC is
not properly set up.

Note that the DCHECK also triggers, without actual consequences, for aborting
incremental marking.

BUG=468240
TBR=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2611753002
Cr-Commit-Position: refs/heads/master@{#42044}