CapturedJSStack::as_string should pass a principal argument

[jdm]

We always pass a null pointer to BuildStackString right now:

mozjs/mozjs/src/rust.rs

Line 1016 in d6c3bd9

ptr::null_mut(),

Equivalent code in Firefox obtains the current context realm and the realm of the stack object and chooses the least privileged of the two (by checking if one subsumes the other). Another option would be to allow the caller to pass an explicit principal and delegate this choice to the embedder, which would (naively) require making this an unsafe method.