Commit() and GrowTo() iterate allocating pages, and call RewindPages()
if an iteration fails. This is true even if the first iteration fails,
in which case there are no pages to rewind, and RewindPages() DCHECKs.

Only call RewindPages() if we are on the second or later iteration.

Bug: chromium:851626
Change-Id: Ifb644416331b5129c679983bc6af0d21c3ce14d8
Reviewed-on: https://chromium-review.googlesource.com/1099605
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53724}