appcache: bump AppCache chrome://flags expiry to M90

Bug: 582750
Change-Id: I915211564079f2619e04fd10c5164ea30f63b663
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415209
Commit-Queue: Chase Phillips <cmp@chromium.org>
Reviewed-by: Chase Phillips <cmp@chromium.org>
Auto-Submit: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808494}

chrome/browser/flag-metadata.json

@@ -136,7 +136,7 @@
   {
     "name": "app-cache",
     "owners": [ "enne" ],
-    "expiry_milestone": 86
+    "expiry_milestone": 90
   },
   {
     "name": "app-service-adaptive-icon",