Safety check on Mobile: extend flag expiration to M90

Bug: 1070620, 1078782
Change-Id: Id0dcd9491446ea8acdd5c752409dff4394c2ad5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2557080
Auto-Submit: Andrey Zaytsev <andzaytsev@google.com>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Reviewed-by: Sean Harrison <harrisonsean@chromium.org>
Commit-Queue: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Andrey Zaytsev <andzaytsev@google.com>
Cr-Commit-Position: refs/heads/master@{#830562}

chrome/browser/flag-metadata.json

@@ -4124,7 +4124,7 @@
       "msramek",
       "chrome-friendly-settings@google.com"
     ],
-    "expiry_milestone": 88
+    "expiry_milestone": 90
   },
   {
     "name": "safety-check-chrome-cleaner-child",
@@ -4143,7 +4143,7 @@
       "msramek",
       "chrome-friendly-settings@google.com"
     ],
-    "expiry_milestone": 88
+    "expiry_milestone": 90
   },
   {
     "name": "safety-tips",