Fix registration of internal plugins broken by rev 23501

Due to the plugin_list changes in rev 23501 internal plugins registered via a call to PluginList::RegisterInternalPlugin() are no longer being added to the plugin list when LoadPlugins() is called. Fix the problem by adding the internal plugins in LoadPlugins(). 

Original change by Marshall Greenblatt <magreenblatt@gmail.com> at http://codereview.chromium.org/173107.

Review URL: http://codereview.chromium.org/208026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26595 0039d316-1c4b-4281-b951-d872f2087c98