similar-origin window agents have the [[CanBlock]] flag set to false.
Achieve this by hooking up JS's concept with an agent to HTML::Agent.
For now, this is only hooked up to the similar-origin window agent
case but should be extended to the other agent types in the future.
Instead of always reporting a colno and lineno of zero try and use the
values from the Error object that may be provided, falling back to the
source location of the invocation if not provided. We can definitely
improve the reporting even more, but this is a start!
Also update this function to latest spec while we're in the area.
The 'reason' was getting initialized to 'empty' state when not
provided through the constructor, which results in a crash when
accessed through throw_dom_exception_if_needed in the generated
IDL getter.
