IPCBlobInputStream must implement nsIIPCSerializableInputStream interface.
When this is done, the child sends the internal ID of the IPCBlobInputStream to
the parent.
An IPCBlobInputStream can be sent back to the parent process (not implemented
in this patch). When this is done, we basically send only the internal ID.
From this ID, we can retrieve the original inputStream because any
IPCBlobInputStreamParent actor has previously registered it into a singleton:
IPCBlobInputStreamStorage.
So, if we have a IPCBlobInputStreamParent, we have an inputStream, and this
inputStream is known by IPCBlobInputStreamStorage. This will be useful in the
next patches.
