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Only for new navigations started with |loadCurrentURLInNativeView|. This
fixes the brief blank screen before NTP is presented. WebStateObserver
callbacks are not moved; they are still only triggered after placeholder
load finishes. This is because navigation manager state (e.g. last
committed item) must only be updated after WKWebView back-forward history
is updated.

Bug: 819606
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ied642aaf0b4fdcab057faea7ee320ad5b5a88478
Reviewed-on: https://chromium-review.googlesource.com/1104301
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568266}