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Currently all metadata is stored in PartitionPage, which is confusing
because the most commonly used metadata is related to slot spans, and
is stored only in the PartitionPage object that corresponds to the first
partition page of the slot span. This CL introduces SlotSpanMetadata
to clarify that confusion.

Change-Id: Id8873dba1c9e3018a8643f4f9c93e694f2edb9c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466007
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817117}