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This patch modifies the wallet datatype to always have a datatype controller,
and give that datatype controller the responsibility of enabling and disabling
the datatype as necessary. If the sync experiment pref is disabled, the datatype
will not start up, and if it's already started will turn itself off. Similarly, if the
experiment is enabled, the datatype is capable of starting itself up.

BUG=470362

Review URL: https://codereview.chromium.org/1024553010

Cr-Commit-Position: refs/heads/master@{#322274}