Adjust freeze-user-agent flag expiration

Bug: 955620
Change-Id: I2572a1b33f0ea6371b65f9e74fab0fe182e11244
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351548Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Aaron Tagliaboschi <aarontag@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797210}

chrome/browser/flag-metadata.json

@@ -2589,7 +2589,7 @@
   {
     "name": "freeze-user-agent",
     "owners": [ "yoavweiss", "aarontag" ],
-    "expiry_milestone": 85
+    "expiry_milestone": 89
   },
   {
     "name": "fullscreen-controller-browser-scoped",