crostini-browser.desktop

[Desktop Entry]
Name=Browser chooser
Exec=/usr/local/bin/crostini-browser.sh %U
MimeType=text/html;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;x-scheme-handler/mailto;
Type=Application
NoDisplay=true
OnlyShowIn=Never

crostini-browser.sh

#!/bin/sh
case "$(zenity --question --title=sensible-browser --icon-name=web-browser --no-markup --ellipsize --text="Open $* in..." --switch --extra-button "Chrome OS" --extra-button "Linux")" in
  "Chrome OS") exec /opt/google/cros-containers/bin/garcon --client --url "$@" ;;
  "Linux") exec /usr/bin/google-chrome-stable "$@" ;;
esac
exit $?

mimeapps.list

[Default Applications]
text/html=crostini-browser.desktop
x-scheme-handler/http=crostini-browser.desktop
x-scheme-handler/https=crostini-browser.desktop
x-scheme-handler/ftp=crostini-browser.desktop
x-scheme-handler/mailto=crostini-browser.desktop