Comment 5 for bug 670128

I now have chromium-browserchromium-browser 9.0.587.0~r66374-0ubuntu1~ucd~dev2 from ppa:chromium-daily-dev, which has already been updated to include
  MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;
in /usr/share/applications/chromium-browser.desktop. However, this isn’t sufficient to cause Chromium to act as the default browser, either when I select it in gnome-default-applications, or when I tell Chromium to make itself the default browser in its Preferences page.

It finally worked after I manually ran
  xdg-mime default chromium-browser.desktop x-scheme-handler/http
  xdg-mime default chromium-browser.desktop x-scheme-handler/https

gnome-default-applications-properties needs to be updated to set the default associations for these new types, and xdg-settings needs to be updated so that chromium can set them for itself.