Issue #1767 - Prevent incorrect calling of network change detection function.

This rewrites the websocket channel network change detection function to not
skip part of its logic in a situation that has already been checked, preventing
a thread race. See analysis of the problem in the issue.

netwerk/protocol/websocket/WebSocketChannel.h

@@ -146,6 +146,7 @@ private:
   void GeneratePong(uint8_t *payload, uint32_t len);
   void GeneratePing();
 
+  nsresult OnNetworkChangedTargetThread();
   nsresult OnNetworkChanged();
   nsresult StartPinging();