Frames should be treated as active mixed content. When the top frame (other browsers check the parent frame) is loaded via HTTPS and a subframe is loaded via HTTP, a mixed content callback should be triggered to allow browsers to warn the user or block the content. It's true that the insecure frame cannot modify the contents of the secure frame, but it should not be possible for a substantial portion of the page to be loaded with no security despite the presence of a browser security indicator.
Chrome, Firefox, and Internet Explorer all block mixed content frames by default .
Created attachment 244905 [details]
Created attachment 245835 [details]
This is fixed by r181134. I'm not going to file a bug for the minor behavioral difference because our way is easier :)