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Since PrefService provides a consistent and convenient way to check if a user
value has ever been saved for a preference, there is no need for
NETWORK_PREDICTION_UNSET in enum chrome_browser_net::NetworkPredictionOptions.
The default value is NETWORK_PREDICTION_WIFI_ONLY, this is reported by
PrefService if there is no user setting, policy, etc.

Note that since MigrateNetworkPredictionUserPrefs was introduced in
https://codereview.chromium.org/421653006/diff/160002/chrome/browser/net/prediction_options.cc,
there is no need to check the old preference NetworkPredictionEnabled in
CanPrefetchAndPrerender* or CanPreresolveAndPreconnect*.

Also note that Chromium at no point did save NETWORK_PREDICTION_UNSET to disk,
it only used to be provided as a default option.  This guarantees that this CL
does not introduce functional change.

TODO:
 * Remove network_prediction_enabled from profile_io_data.
 * Remove PrerenderCondition classes.

As this CL will also fix UI bug https://crbug.com/404794, I prefer to land it
first, and deal with the other two clean-up items on a separate CL.

BUG=406583

Review URL: https://codereview.chromium.org/516443002

Cr-Commit-Position: refs/heads/master@{#292613}