Show the install dialog for the initial load of an unpacked extension with plugins.

      BUG=83273
TEST=in the extensions managmenet page, with developer mode enabled, Load an unpacked extension on an extension with NPAPI plugins. You should get an install dialog.

Review URL: http://codereview.chromium.org/7077018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@87637 0039d316-1c4b-4281-b951-d872f2087c98