[AF] Extend AutofillEnableAccountWalletStorage flag expiry

This needs to be enabled by default only for a specific set of 50
countries, which we'll accomplish via the VariationsService.
Until then, it should not expire.

Bug: 983940
Change-Id: I600269299eed0ecc63be7642171161928182975c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1745550
Auto-Submit: Jared Saul <jsaul@google.com>
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685503}

chrome/browser/flag-metadata.json

@@ -880,8 +880,8 @@
   },
   {
     "name": "enable-autofill-account-wallet-storage",
-    "owners": [ "feuunk", "butter-team@google.com" ],
-    "expiry_milestone": 76
+    "owners": [ "treib", "jsaul@google.com", "butter-team@google.com" ],
+    "expiry_milestone": 81
   },
   {
     "name": "enable-autofill-credit-card-ablation-experiment",