Disable ExternalExtensionAutoAcknowledgement on Win

BUG=309833
TBR=miket@chromium.org
CC=garykac@chromium.org

Review URL: https://codereview.chromium.org/33423003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230019 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230019