use it to implement ExtensionInstallUI.

There is (still) no dialog here. But the next CL will, um,
definitely have it.

Also, fixed the issue with theme preview infobars not updating if you install a theme while another one is already previewed.

BUG=17932

Review URL: http://codereview.chromium.org/160483

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@22228 0039d316-1c4b-4281-b951-d872f2087c98