[Safety Tips] Punt expiration of flag by one milestone.

Fixed: 1137858
Change-Id: I7eefc389cce69d1cd5fd271ab1009ab4faade4ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468336
Auto-Submit: Joe DeBlasio <jdeblasio@chromium.org>
Commit-Queue: Chris Thompson <cthomp@chromium.org>
Reviewed-by: Chris Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816672}

chrome/browser/flag-metadata.json

@@ -4054,7 +4054,7 @@
   {
     "name": "safety-tips",
     "owners": [ "jdeblasio", "estark", "meacer" ],
-    "expiry_milestone": 87
+    "expiry_milestone": 88
   },
   {
     "name": "same-site-by-default-cookies",