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|PartitionAllocGetSlotOffset| currently returns the offset from the
beginning of the user data area rather than the actual slot start.
Remove unnecessary pointer adjustments to make the implementation
match the name.

Similarly, update |PartitionAllocGetSlotStart|.

Bug: 1073933
Change-Id: I82bcd3a397ab01f1a25ae98b0b768b9bd81315a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613050
Commit-Queue: Sergei Glazunov <glazunov@google.com>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841056}