Initialize PasswordProtectionRequest::request_outcome_ to RequestOutcome::UNKNOWN

In tests, the request mode is bound to the close callback in
safe_browsing::ChromePasswordProtectionService::ShowModalWarning().
However, request mode is never initialized in tests, presumably because
PasswordProtectionService::StartRequestForTesting() does not result in
running the typical safe browsing flow.

Unfortunately, passing an uninitialized value counts as a use for the
purposes of C++, and using unininitialized values is undefined behavior.
This was never detected in the past, because MSan only implemented these
stricter checks with `msan_eager_checks=true`.

Bug: 340729470
Change-Id: Iecf7a9304844704e8601046459b9e6f3033130ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5539832
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1301372}