Disable precache on svelte.

Disable precache on svelte.

Precache was enabled for svelte in http://crrev.com/c8e7c272, when the
experiment was only on dev, in the interest in seeing the maximum percent of
users that we could impact.

There is an unconfirmed possibility that the background service may slow down
phones with low RAM/CPU. Until we verify that it does not, we would like to
avoid launching on svelte devices, in order to eliminate any potential for
harm.

This change re-disables the service on svelte devices, by checking
PrivacyPreferencesManager#shouldPrerender() instead of
PrefServiceBridge#getNetworkPredictionEnabled().

BUG=683259
Review-Url: https://codereview.chromium.org/2642733004
Cr-Commit-Position: refs/heads/master@{#445110}
(cherry picked from commit 5fe1163de91a80f23c4a2b68ca1d1a5c53319986)