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This allows session restore to use the disk cache for prefetched search
navigations (it also supports cross-session forward back). The pref is
lossy since it would not matter too much to miss these cases
occasionally, and the pref is updated somewhere around 1% of
navigations.

Bug: 1162121
Change-Id: I967a0cd784ad9b9768cac84878a9b683cbd698ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613357
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841269}