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Currently pages are committed as soon as the slot span is allocated.
There is already code that cleverly avoids writing in-slot free list
entries on pages of the span that aren't used yet, to prevent them from
being faulted in. This code can be easily repurposed to commit pages
as needed. An adjustment had to be made to not provision a slot (and add
it to the free list) if it doesn't fully fit in a committed page
(previously it was enough for the slot's free list entry to fit within
the page).

Change-Id: Ic9585aa6001a1cf53233ae5edc4932be780a93ad
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2563385
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833127}