Move SyncPolicyHandler to live in chrome/browser/sync.

This facilitates the refactoring of chrome/browser/policy into a layered
component.

BUG=271392
R=atwilson@chromium.org

Review URL: https://codereview.chromium.org/39903003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230978 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230978