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AllocNewSlotSpan allocates kSuperPageSize memory if there is no available pages,
but returns only 1 slot span. So AllocNewSlotSpan should make 1 slot span accessible.
(the slot span size is provided by get_bytes_per_span()).

This patch also fixes total_size_of_committed_pages.
For normal buckets, total_size_of_committed_pages are updated in the following way:
- PartitionBucket::AllocNewSlotSpan increases by get_pages_per_slot_span()*PartitionPageSize().
- PartitionBucket::SlowPathAlloc increases by get_bytes_per_span() when recommitting pages.
- PartitionPage::Decommit decreases by get_bytes_per_span().

get_bytes_per_span() is not always equal to get_pages_per_slot_span()*PartitionPageSize(), because
get_pages_per_slot_span() is a natural number of PartitionPages (calculated by get_system_pages_per_slot_span()).
E.g. num_system_pages_per_span is 3 (0 < num_system_pages_per_span < MaxSystemPagesPerSlotSpan()).

Change-Id: Ie46c9099086bd88afa6ee9f654cd98f21935d493
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444671
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814119}