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On Windows, ImportantFileWriter boosts its thread's priority for the
moment it calls ::ReplaceFileW in the hopes that this will improve its
chances of beating other processes (e.g., A/V scanners) that will try to
open the files it needs. This change preserves the last error code
coming out of a failure of ::ReplaceFileW for use in logging and metrics
reporting.

BUG=1145235
R=jdoerrie@chromium.org

Change-Id: I1e275698c5c39fa7cfa97d6b3bf48e2023a8763f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2556841
Commit-Queue: Greg Thompson <grt@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830557}