opening index.html via file: does not work on chrome/safari

[r10s]

the communication between simulated peers do work on firefox using localhost or file:.

for chrome/safari, this only works for localhost currently.

check, if we can improve that, otherwise document the issue.

[r10s]

reason seems to be different same-origin-policies used on firefox and chrome/safari:

• firefox treats windows using the same file: url as the same origin
• chrome/safari treat windows using the same file: url as different origins

only windows with the same origin can access the other window objects as we're doing currently. postMessage seems not be able to fix that, however, localStorage works.