Additional string for protections against forced extension installs

This is for the case where the dialog might overflow and we have to
say "... and N more ..."

TBR=finnur
BUG=315192

Review URL: https://codereview.chromium.org/59883003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233052 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233052