Add hook for extension to check sync status.

Adds a new function to preferencesPrivate that returns a list of
unencrypted sync categories. Also changes preferencesPrivate to use a
whitelist instead of being restricted to component extensions.

BUG=326678

Review URL: https://codereview.chromium.org/118413003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241977 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241977