Bug 983497 - Patch 1: Infrastructure to fire events on ServiceWorkerContainers. r=ehsan,smaug

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extra : transplant_source : S1z%F3%86%E5%CF%D6%3BWW%8E%1A%90%A3%94%7E%FE%E3z
Bug 983497 - Patch 2: Fire updatefound on ServiceWorkerContainers. r=ehsan

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extra : transplant_source : %98%1B%D73y%CF%A9%3Fb%DAR%DC%25%A9S%15%22%CF%C4%1B