Some code review changes.

src/constants.js

@@ -0,0 +1,8 @@
+// Video call states.
+export const CALL_STATE_NONE = 0;
+export const CALL_STATE_OUTGOING_INITATED = 1;
+export const CALL_STATE_INCOMING_RECEIVED = 2;
+export const CALL_STATE_IN_PROGRESS = 3;
+
+// Mime type used for video call messages.
+export const CALL_MESSAGE_MIME_TYPE = 'application/x-tinode-webrtc';