Move PreferencesMac to components/policy/.

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

BUG=271392

Review URL: https://codereview.chromium.org/78603003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237409 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237409