When this is done, can assign to lpalmaro to try it

+lpalmaro@ for guidance.
What I want to do is to have 3 containers (as defined in the voice over rotor):
- The top toolbar
- The WebPage
- The bottom toolbar

The user can navigate quickly from one toolbar to the other using those containers navigation. But using the standard "next item", the user would go through all the elements of the top toolbar, then all the elements of the page, then all the elements of the bottom toolbar.
This is what Safari and other app provided by Apple are doing.

Is that what a user used to voice over would expect?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/74675afc3475450a2391eb235004bc7169538a5d

commit 74675afc3475450a2391eb235004bc7169538a5d
Author: Gauthier Ambard <gambard@chromium.org>
Date: Thu Jun 28 14:30:09 2018

Add voice over "container" behavior

This CL adds accessibility traits to the toolbars such as they are
acting as VoiceOver "container". If the user sets the accessibility
rotor to "containers", the user is able to go navigate quickly between
the toolbars and the webpage.

Bug:  856212 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I309a1308bb200bf9f90aaa4b164a3798c93c0f31
Reviewed-on: https://chromium-review.googlesource.com/1118275
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571111}
[modify] https://crrev.com/74675afc3475450a2391eb235004bc7169538a5d/ios/chrome/browser/ui/toolbar/adaptive/adaptive_toolbar_view_controller.mm