Today we only support one MediaCryptoReadyCB at a time in MediaDrmBridge. This works today because audio and video decoders are initialized sequentially, so we will at most have one MediaCryptoReadyCB at any time. But this is related to media pipeline's initialization process in the Render process and it's really not obvious given that MediaDrmBridge is running in the GPU process. We should just support multiple MediaCryptoReadyCB callbacks so that we don't rely on implementation detail of media pipeline.