[cr133] GetOfferForUrl returning a const T*

As `BraveIsOfferValid` was taking a multable pointer, the change to a
const pointer caused a build breakage.

Chromium change:
https://chromium.googlesource.com/chromium/src/+/4ee9d87d53be7f3423cc433f769f09f9b33c62af

commit 4ee9d87d53be7f3423cc433f769f09f9b33c62af
Author: Jan Keitel <[email protected]>
Date:   Tue Dec 17 01:19:36 2024 -0800

    Let PayDM::GetAutofillOffers return vector of const pointers.

    As a drive-by, only pass PayDM to the AutofillOfferManager. It does
    not need the full PersonalDataManager.

    Bug: 40100455

chromium_src/components/autofill/core/browser/payments/offer_notification_handler.cc

@@ -14,7 +14,7 @@ namespace {
 
 // This replicates the functionality that the removed upstream flag
 // kAutofillEnableOfferNotificationForPromoCodes used to have.
-bool BraveIsOfferValid(AutofillOfferData* offer) {
+bool BraveIsOfferValid(const AutofillOfferData* offer) {
   if (!offer) {
     return false;
   }