Remove last extension related code in content\browser. Have the ExtensionService manage what to do when extensions with plugins are installed, as opposed to in PluginService which being in content shouldn't know about extensions.

      BUG=76789
Review URL: http://codereview.chromium.org/6720053

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@80528 0039d316-1c4b-4281-b951-d872f2087c98