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After https://codereview.chromium.org/197013007
ConfigurationPolicyHandler provided a default implementation of ApplyPolicySettings() that simply called NOTREACHED.
Also it wasn't clear anymore what of ConfigurationPolicyHandler is the public interface and what is default implementation.

Both issues are resolved with this change.

Review URL: https://codereview.chromium.org/1019283005

Cr-Commit-Position: refs/heads/master@{#322381}