[iOS] Adding ui/menu OWNERS file

Listing gambard and myself as co-owners of this directory.

Bug: 1093302
Change-Id: I2e37ebeb2c5ffe4cb649026a443e0707df06f1d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2323131Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Sebastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792699}

ios/chrome/browser/ui/menu/OWNERS

+gambard@chromium.org
+seblalancette@chromium.org
+
+# TEAM: ios-directory-owners@chromium.org
+# OS: iOS