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If GigaCage isn't enabled, this DCHECK will get triggered:
{FreeNoHooksImmediate,PartitionAllocFreeForRefCounting}->
->PartitionRefCountPointer->DCheckGetSlotOffsetIsZero->
->PartitionAllocGetSlotOffset->
->DCheckIfManagedByPartitionAllocNormalBuckets

GigaCage isn't needed to set up PartitionRefCount. It's only needed to
easily detect from BackupRefPtr code if PartitionRefCount is available,
but this won't be an issue because BackupRefPtr is disabled when
GigaCage isn't needed.

Bug: 1073933
Change-Id: I78faea9427ca06643033d0025cdef6472131c816
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2621855
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842861}