Update AppService Shelf integration flag expiry_milestone to M81.

BUG=998004

Change-Id: I0428b84e59d719be9ce7288a11094b5bb74ee3d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918904Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715609}

chrome/browser/flag-metadata.json

@@ -135,7 +135,7 @@
   {
     "name": "app-service-shelf",
     "owners": [ "//chrome/services/app_service/OWNERS" ],
-    "expiry_milestone": 79
+    "expiry_milestone": 81
   },{
     "name": "arc-application-zoom",
     "owners": [ "sarakato" ],