[iOS] Add toolbar_container/OWNERS file.

The owners are kkhorimoto@ (original author), and gambard@
(reviewer).

Bug: 880672
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I2d099c944b12e77c04106dc6de6de428824f1272
Reviewed-on: https://chromium-review.googlesource.com/c/1220720Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599970}

ios/chrome/browser/ui/toolbar_container/OWNERS

+gambard@chromium.org
+kkhorimoto@chromium.org