[iOS] Added OWNERS file to sharing directory.

Reasons for becoming an owner:
- I created this directory in: crrev.com/c/2297601
- The creation of the SharingCoordinator stems from a non-trivial
  design requirement to support showing an ActivityView at an arbitrary
  position in the layout tree. Since I designed this solution, it makes
  sense that I keep an eye on contributions made to it.

Change-Id: I229db494287a138ababa3437acdde5119028d283
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472198Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Sebastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817519}

ios/chrome/browser/ui/sharing/OWNERS

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