Browsers are weird. If an iframe has a `name` attribute, it’s window
will be cached. When you click a link, then go back, it will insert the
**same** src window into the iframe regardless of a different `src`.

Well, that hurts us since we generate a unique domain to load 3p
frames. We’ll continue to use an old domain’s window, even though the
src points to a new domain. Worse, when we post messages to the 3p
frame, we will send it with an origin parsed from the `src` attribute
even though that is not the real URL of the frame’s window.

Fixes ampproject#2943