Do not protect extensions.known_disabled

Even if it gets mucked with; clearing it just makes it worse...

Review URL: https://codereview.chromium.org/146023006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249677 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249677