1. DomName('StorageUsageCallback')
  2. Experimental()
void StorageUsageCallback(int currentUsageInBytes, int currentQuotaInBytes)

Source 

@DomName('StorageUsageCallback')
// http://www.w3.org/TR/quota-api/#idl-def-StorageUsageCallback
@Experimental()
typedef void StorageUsageCallback(int currentUsageInBytes, int currentQuotaInBytes);