Extend flag animated-avatar-button to M81

Bug: none
Change-Id: I7ee732c1c123ee1a18fa51adf235ccbd5d3efe1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899494
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712535}

chrome/browser/flag-metadata.json

@@ -105,7 +105,7 @@
   {
     "name": "animated-avatar-button",
     "owners": ["jkrcal"],
-    "expiry_milestone": 79
+    "expiry_milestone": 81
   },
   {
     "name": "app-management",