[PA] Add another PartitionPage into PartitionDirectMapMetadata

Otherwise SlowPathAlloc() overwrites PartitionDirectMapMetadata::bucket
when calling SetRawSize(), which reaches to the next PartitionPage after

PartitionDirectMapMetadata: :page via pointer arithmetic.
Change-Id: I0ca8a92ddf10f0687cb7f704148e283f0d748bba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2914964


Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/master@{#886220}