If we just naively use the thread manager to dispatch the event via SpecialPowers,
we cannot avoid the XPCWrappedJS for the runnable being in chrome scope (even if
we pass a content object, we make the call in chrome scope, so we pass a cross-
compartment wrapper). This means that the machinery in
nsXPCWrappedJSClass::CheckForException ends up calling the error reporter for the
SpecialPowers scope, which isn't what we want.
Messing around with the implementation of CheckForException is playing with fire.
So we work around this with an explicit Cu API. :-(
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