[iOS][MF] Pause and restore keyboard custom view

This CL adds the capability to pause the keyboard views when the
webstate is hidden. This happens i.e. when the passwords coordinator
presents a full screen selection. This way, when the webstate is shown
again, it can restore it's views if needed.

Bug: 845472
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I5eddf31ada277a1a9c0c0e8cc0ae7be3d8d01ad2
Reviewed-on: https://chromium-review.googlesource.com/c/1236436
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Reviewed-by: Eric Noyau <noyau@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596648}
12 files changed