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Also added NavigationManagerImpl::IsRestoreSessionInProgress method, so
WebStateImpl knows about in-progress restoration and can behave
accordinly.

Bug: 877671
Change-Id: Ic16391b8ba6a2a5742dee7c3fe91bc05c79d25fc
Reviewed-on: https://chromium-review.googlesource.com/c/1352413Reviewed-by: Danyao Wang <danyao@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611725}