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This enables callers to avoid doing it themselves (centralizing the `memset` in
1 place, rather than duplicating it at every call site that wants it), and to
allow the PartitionAlloc implementation to avoid doing it itself if the
operating system has already zero'd the new page(s).

Bug: 864462,680657
Change-Id: Ibeae0bf7b9f20238334bf9bc022d0f243a77ef11
Reviewed-on: https://chromium-review.googlesource.com/1162825
Commit-Queue: Chris Palmer <palmer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589389}