[Australis] Bug 987177 - invalidate wrapper's node reference, r=jaws, a=sylvestre

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extra : transplant_source : W8F%29%7D%BA%E0%864%B8%9Eu%96%CE%A0%EB%3Bh6%81
[Australis] Bug 987177 - make destroyWidget clear caches for XUL wrappers, r=jaws, a=sylvestre

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extra : transplant_source : %23%0E%FB%F7%F5%8D%8Fr%87%97%A9%5E%1D%D7K%E5%A8%14%B7%0E