GitLab

This CL adds a switch used to change the positions of the toolbars
buttons, allowing to test different layouts.

The 3 layouts possible are:
- Navigation controls in the bottom and nothing in top toolbar (default)
- Navigation controls in the bottom and share button in the top toolbar
- Navigation controls in the top toolbar and bookmark/share in the
   bottom toolbar

Those layout will mainly change the visibility of the different buttons.
Also the second forward button is added to the toolbar views.

Bug: 824175
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I8e76b081038fb444bdac8f9e823dc023b4b9e883
Reviewed-on: https://chromium-review.googlesource.com/973609
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Eric Noyau <noyau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545015}