Bug 85447

The [V8EnabledAtRuntime=encryptedMedia] markup in the IDL prevents the Encrypted Media Extensions methods from being called because they are not available to JavaScript. However, at least one of the events and the new error could be reported by the port without the page initiating it. Specifically, a needkey event may be fired if the port detects and encrypted file. Also, it might report MEDIA_ERR_ENCRYPTED. Rather than modify the behavior of the port, which require passing encryptedMediaEnabled() to various layers, we should prevent the new events and error from being reported. It should be safe to just drop the event and convert MEDIA_ERR_ENCRYPTED to MEDIA_ERR_DECODE since decode is where encrypted content would cause a failure.