Perform deep copy of NavigationItems for serialization.
Perform deep copy of NavigationItems for serialization.

This fixes the race condition in which NavigationItems that were
constructed on the main thread are being deallocated on the IO thread
during session serialization.

BUG=none

Review URL: https://codereview.chromium.org/1292143004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#344059}

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