Check that a preference is registered when setting its policy.
Check that a preference is registered when setting its policy.

BUG=None
TEST=policy.py

Change-Id: I7f6480af928827fc6d4ead734f93c68bdad6f6ec

Review URL: https://chromiumcodereview.appspot.com/9934003

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@129710 0039d316-1c4b-4281-b951-d872f2087c98

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