PartitionAlloc: Initialize members of a newly created PartitionSuperPageExtentEntry
PartitionAlloc: Initialize members of a newly created PartitionSuperPageExtentEntry

Also this CL cleans up logic to link the newly created PartitionSuperPageExtentEntry
to a linked list.

BUG=

Review URL: https://codereview.chromium.org/1043813002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@192786 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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