Update flag-metadata.json for allow-sxg-certs-without-extension

Bug: 851778,862003
Change-Id: If6667d8b8f4813748fdd1bceb40850bdfd5da36f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1573431
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652519}

chrome/browser/flag-metadata.json

@@ -71,7 +71,9 @@
   {
     "name": "allow-sxg-certs-without-extension",
     "owners": [ "//content/browser/web_package/OWNERS" ],
-    "expiry_milestone": 76
+    // Used by developers for testing signed exchange loading using normal
+    // certs. See https://crbug.com/862003
+    "expiry_milestone": 83
   },
   {
     "name": "android-files-in-files-app",