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The PrefManager class owns an IO Object (CacheObserver) and
a Pref object (PrefDelegate).

The embedder would need to implement PrefDelegate and create
an instance of PrefManager for prefs to work. Right now, only
writes to the prefs is implemented. Reads would come in the
next CL, along with the Chromium implementation.

BUG=490870

Review-Url: https://codereview.chromium.org/2322183002
Cr-Commit-Position: refs/heads/master@{#420786}