[Passwords] Bump up expiration of UsernameFirstFlow flag

Bug: 959776
Change-Id: Id86d44b6fd42a06a072f73381ecc78b4a5905336
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627159
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Friedrich [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843131}

chrome/browser/flag-metadata.json

@@ -4945,7 +4945,7 @@
   {
     "name": "username-first-flow",
     "owners": [ "jdoerrie" ],
-    "expiry_milestone": 88
+    "expiry_milestone": 92
   },
   {
     "name": "video-player-js-modules",