Elements tab crashes with NPE in DOM.ProviderListener.onChildRemoved()

[rickbrew]

In onChildRemoved, we try to get the nodeId for an element that isn't mapped. We end up with a null Integer and then blindly try to store it into Node's nodeId field (an int), and immediately NPE.

This happened several times, but the stack trace has long since scrolled off into oblivion and I can't repro it anymore. For now I've got a fix that logs a warning and skips sending the DOM.childNodeRemoved event over the wire.