Add Web.RestoreNavigationItemCount UMA histogram.

Logs the number of items Navigation Manager was requested to restore.
100 is logged when the number of navigation items is greater than 100.
This is just a requested count and actual number of restored items can
be smaller.

Bug: 877671
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Id528320cfae35a04c9199d21ff14d5ff72c0aed2
Reviewed-on: https://chromium-review.googlesource.com/1225697
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Danyao Wang <danyao@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591532}
8 files changed