[iOS] Upstream OWNERS files

Add a wildcard rule in ios/OWNERS to allow anyone to review *.gyp changes
when only adding or removing files.

BUG=None

Review URL: https://codereview.chromium.org/1137823003

Cr-Commit-Position: refs/heads/master@{#329644}

ios/OWNERS

 droger@chromium.org
 rohitrao@chromium.org
 stuartmorgan@chromium.org
+
+# per-file rules:
+# These are for the common case of adding or renaming files. If you're doing
+# structural changes, please get a review from a reviewer in this file.
+per-file *.gyp=*