The specification has a list of rules about when [simple dialogs cannot
be shown][u1]. This change implements that part of the specification and
also the bits that specify when to noramlize newlines in their messages.
In addition, it fills in the remaining specification step comments for
these methods.
[u1]: https://html.spec.whatwg.org/multipage/#cannot-show-simple-dialogs
Testing: This causes some CSP tests run via servodriver to start
passing.
Unexpected alert dialogs cause failures in servodriver.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>