The old tab sync favicon experiment is deprecated. This patch updates the
strings to the fact that we now have a separate datatype, and hooks this
up to a new sync field.
BUG=154886
Review URL: https://chromiumcodereview.appspot.com/12700016
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@188974 0039d316-1c4b-4281-b951-d872f2087c98