Joint Session history seems not to update when "src" of iframe changes

[gterzian]

When writing this test, I noticed that I had to use frame.contentWindow.location = "/common/blank.html"; and that a manipulation of the src of the iframe didn't seem to have an effect on the session history.

When manipulating the src, the call to frame.contentWindow.history.go(-1); would result in hitting

servo/components/constellation/constellation.rs

Line 2511 in 481662d

return warn!("Cannot traverse that far into the past.");

cc @cbrewster