Remove hardcoded Windows registry keys in PolicyLoaderWin stuff.

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

BUG=271392

Review URL: https://codereview.chromium.org/68553003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236248 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236248