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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}