I'll make the change to also install Chrome handling for gnome-www-browser for completeness...but it won't fix this problem. Even if I set that up...and then in that case garcon still handles gnome-www-browser, you can still reproduce the problem. The issue is that the desktop file for firefox registers the mime types for http/https as well...and since it was installed after garcon, firefox will appear first in the mime type list. And gtk relies on that mime type list to determine which app to use.

But IMO...if you install a browser in the container, then you're sort of expecting that to be the browser that's used.  xdg-open will also use firefox in this case as well.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/containers/cros-container-guest-tools/+/32f75d96cbf4c4581a8bf4f994a6746d6307257c

commit 32f75d96cbf4c4581a8bf4f994a6746d6307257c
Author: Jeffrey Kardatzke <jkardatzke@google.com>
Date: Wed May 02 21:12:39 2018

Register as gnome-www-browser and add desktop-file-utils dep

This registers the garcon URL handler as the gnome-www-browser
alternative. It also adds the desktop-file-utils package as a dependency
for garcon. That package is needed in order to do updates to the
/usr/share/applications/mimeinfo.cache file which GTK relies on for
resolving URL handling. (I apparently had that installed when I tested
all of this before)

BUG= chromium:839070 
TEST=Verified works with minimal install in container

Change-Id: I26b5b10643d5e557036418c33768985a61fde971
Reviewed-on: https://chromium-review.googlesource.com/1040727
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Tested-by: Jeffrey Kardatzke <jkardatzke@google.com>

[modify] https://crrev.com/32f75d96cbf4c4581a8bf4f994a6746d6307257c/cros-garcon/postinst
[modify] https://crrev.com/32f75d96cbf4c4581a8bf4f994a6746d6307257c/cros-garcon/BUILD