[iOS] Move OpenIn to its own directory

This CL moves the OpenIn files to their own directory, cleaning up the
main ios/chrome/browser/ui directory.

Bug: 944008
Change-Id: I46b3aaaa27e1b1e1e97388732dc52535b884e7e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530778
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642919}
14 files changed