[ios] Add flag ownership for rkgibson

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: I5a3ce15915d122ee6c12d2f5aaf9bc559758c4a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095200Reviewed-by: Robbie Gibson <rkgibson@google.com>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748743}

chrome/browser/flag-metadata.json

@@ -3908,6 +3908,11 @@
     "owners": [ "davidbienvenu", "fdoray" ],
     "expiry_milestone": 84
   },
+  {
+    "name": "webpage-text-accessibility",
+    "owners": [ "rkgibson" ],
+    "expiry_milestone": 86
+  },
   {
     "name": "webui-a11y-enhancements",
     "owners": [ "aee" ],