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I'm going to build a general persistence mechanism for //net instead of
tediously plumbing out to the profile's prefs (which gets really messy
with isolated app contexts and such), so I'm removing the old
persistence stuff until that's done.

BUG=704259

Review-Url: https://codereview.chromium.org/2835923005
Cr-Commit-Position: refs/heads/master@{#467862}