[iOS] Move omnibox popup files into a separate directory.

Moves popup-related classes into i/c/b/ui/omnibox/popup and performs
necessary changes to BUILD.gn files.

Bug: None
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I81bd232799092160463065d874328b015b5d8a8e
Reviewed-on: https://chromium-review.googlesource.com/929141
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540852}
25 files changed