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Also embellish the PA API a bit to make accessing the new zero-fill
functionality more usable.

There are likely additional call sites that can make use of this functionality.

Bug: 864462
Change-Id: I8881c5774b6083b210f5160d4b4f2ac345f2467e
Reviewed-on: https://chromium-review.googlesource.com/1212443
Commit-Queue: Chris Palmer <palmer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590081}