Disallow back forward navigations during async session restoration.

Slim Navigation Manager asynchronously restores the session. During
session restoration it's not possible to perform back-forward
navigation. This change prevents all delta navigations until the session
is restored.

Bug: 877671
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I801a99182582118862d03796532588f1495de9c6
Reviewed-on: https://chromium-review.googlesource.com/1187677
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586041}
2 files changed