Add the not-from-webstore link to the extension settings page.

BUG=320426

Review URL: https://codereview.chromium.org/96843002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237931 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237931