[Windows] Fix error return from RunDeElevated

On error, base::win::RunDeElevated was returning an invalid
base::Process and relying on the Windows last error code being useful to
callers. While cleaning up the callstack on exit, it just so happens
that privilege restoration (specifically, the call eventually made to
LookupPrivilegeValue) was overwriting the error code with ERROR_SUCCESS.

To mitigate this, we change the result of RunDeElevated to
base::expected so that the error code is explicitly returned by the
function in case of error.

Bug: none
Change-Id: Icc2b15990eab3f9f9365199851d53638c2ae559f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6678731
Commit-Queue: S Ganesh <ganesh@chromium.org>
Reviewed-by: S Ganesh <ganesh@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1479336}