Introduce a temporary workaround to ensure tracked preferences set to empty list/dict are not confused with those that are simply not set (i.e., NULL).

Review URL: https://codereview.chromium.org/79013003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236815 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236815