[ios] Add flag ownership for open-downloads-in-files.app

We're adopting Chrome Flag Ownership in iOS. See announcement here:
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: I76c553958e137f6d389433a5e96cb566a1a17f1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097460Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749172}

chrome/browser/flag-metadata.json

@@ -3108,6 +3108,11 @@
     "owners": [ "mtlieuu", "offline-dev@chromium.org" ],
     "expiry_milestone": 76
   },
+  {
+    "name": "open-downloads-in-files.app",
+    "owners": [ "ewannpv", "gambard" ],
+    "expiry_milestone": 86
+  },
   {
     "name": "overlay-new-layout",
     "owners": [ "donnd", "jinsukkim", "contextual-search-eng" ],