[ios] Add flag ownership for jlebel

iOS flag owners were asked to claim ownership and state an expiry in
this announcement email:
https://groups.google.com/a/google.com/g/bling-team/c/7YC_3l6yEzo/m/1-iZg1fPAAAJ

This is one of several CLs that adds iOS flag ownership information into
flag-metadata.json.

The owner of these flags was identified via git blame. A placeholder
expiry milestone of M86 was added. Please either LGTM, or suggest a new
owner and/or expiry.

Bug: 1058614
Change-Id: Ie656f4ce74ad4dbaee0272ff6297b2d107440400
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094247Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748762}

chrome/browser/flag-metadata.json

@@ -2329,6 +2329,11 @@
     // have no access to commandline flags.
     "expiry_milestone": -1
   },
+  {
+    "name": "force-startup-signin-promo",
+    "owners": [ "jlebel" ],
+    "expiry_milestone": 86
+  },
   {
     "name": "force-text-direction",
     "owners": [ "orinj", "pbos", "ellyjones" ],