Unfortunately new "Runtimes" are also created from Runtime::WebContentsCreated which is overridden 
WebContentsDelegate method. Description for it:
  // Notifies the delegate about the creation of a new WebContents. This
  // typically happens when popups are created.

At this stage I do not want to touch "Runtime" class a lot, so this "Observer" is a kind of work around, but I guess comment is needed.