Fix installer download for apps v2.

Fix installer download for apps v2.

This is a poor-man's solution to unblock the apps v2 release. It causes an
existing Chrome window (or a new one if there aren't any) to download the
installer. This is needed because the v2 app doesn't have a brower context,
which means no download shelf for the user to click when the download completes.
The UX is pretty horrible because the Chrome window pops on top of the app
window, typically obscuring the instructions. I'm open to better solutions.

This also fixes a bug in the host setup and install dialogs, which were not
showing the translucent screen that normally obscures the main UI while a
modal dialog is open.

Review URL: https://codereview.chromium.org/282113002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270802 0039d316-1c4b-4281-b951-d872f2087c98