Fix expiration milestone for Quiet Notification Prompts.

Bug: 1123026
Change-Id: Ia12abb01c45cfb1897821bd42f4c4236f95bf1e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380658Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Illia Klimov <elklm@google.com>
Cr-Commit-Position: refs/heads/master@{#802745}

chrome/browser/flag-metadata.json

@@ -3796,7 +3796,7 @@
   {
     "name": "quiet-notification-prompts",
     "owners": [ "engedy", "mkwst", "andypaicu", "hkamila" ],
-    "expiry_milestone": 86
+    "expiry_milestone": 90
   },
   {
     "name": "raw-clipboard",