Revert "kiosk: remove check for a non-guest web contents"

This reverts commit 80a5d677f55d07cee1afbaa0cf39964b00643d96.

Reason for revert: causes unexplained crash for Mime Handler Guest Views b/370099097
Original change's description:
> kiosk: remove check for a non-guest web contents
>
> No need to check a web contents for being nullptr or non-guest,
> since this check is enforced earlier by the caller.
>
> Bug: b/354925677
> Test: compile
> Change-Id: If6e1d83c36071004dd35d79bb04c0a806bff2d91
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5735139
> Reviewed-by: Edman Anjos <edman@chromium.org>
> Commit-Queue: Polina Bondarenko <pbond@chromium.org>
> Auto-Submit: Polina Bondarenko <pbond@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1333402}

Bug: b/354925677
Change-Id: I52950b7ea869cf98cec202890c518e47ed22a1d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5891440
Auto-Submit: Polina Bondarenko <pbond@chromium.org>
Reviewed-by: Edman Anjos <edman@chromium.org>
Commit-Queue: Polina Bondarenko <pbond@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1362333}