Fixed leak of handles at shutdown of the PolicyLoaderWin.

The windows policy loader observes policy update events by invoking
RegisterGPNotification. This change adds calls to
UnregisterGPNotification at shutdown, which release the internal
handles created by the first call.

BUG=315224
R=pastarmovj@chromium.org

Review URL: https://codereview.chromium.org/59853005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233603 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233603