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This reverts commit 101b5ae0.

Reason for revert: Needs some more work, some last minute
refactoring introduced some crashes.

Original change's description:
> [ios] Enable preload with slim-navigation-manager.
> 
> Previously, preload required CopyStateFromAndPrune and
> CanPruneAllButLastCommittedItem within NavigationManager, but those
> are not supported with SlimNav. Here, preload is implemented by
> creating a preload WebState with navigation history by restoring the
> current WebState.
> 
> The performance of this implementation depends on Slim
> Navigation Manager's session restoration performance which can be
> measured with existing kRestoreNavigationTime metric.
> 
> Original implementation of this CL was written by eugenebut@,
> simply updated in this resurrected CL.
> 
> Bug: 834116
> Change-Id: I86c9ecf7e3ed213ebaeddce6264dff3bbb8a8c94
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815702
> Commit-Queue: Justin Cohen <justincohen@chromium.org>
> Reviewed-by: Rohit Rao <rohitrao@chromium.org>
> Reviewed-by: Eugene But <eugenebut@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#701228}

TBR=rohitrao@chromium.org,justincohen@chromium.org,eugenebut@chromium.org,gambard@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 834116
Change-Id: I16beb96bc2d1eab9a9c36a8e03728fd3cc23f035
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1833743Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701746}