--- a/dom/messagechannel/MessagePortChild.cpp
+++ b/dom/messagechannel/MessagePortChild.cpp
@@ -6,16 +6,21 @@
 #include "MessagePortChild.h"
 #include "MessagePort.h"
 #include "mozilla/dom/MessageEvent.h"
 #include "mozilla/ipc/PBackgroundChild.h"
 
 namespace mozilla {
 namespace dom {
 
+MessagePortChild::MessagePortChild()
+  : mPort(nullptr)
+{
+}
+
 mozilla::ipc::IPCResult
 MessagePortChild::RecvStopSendingDataConfirmed()
 {
   MOZ_ASSERT(mPort);
   mPort->StopSendingDataConfirmed();
   MOZ_ASSERT(!mPort);
   return IPC_OK();
 }

--- a/dom/messagechannel/MessagePortChild.h
+++ b/dom/messagechannel/MessagePortChild.h
@@ -14,17 +14,17 @@ namespace dom {
 
 class MessagePort;
 
 class MessagePortChild final : public PMessagePortChild
 {
 public:
   NS_INLINE_DECL_REFCOUNTING(MessagePortChild)
 
-  MessagePortChild() : mPort(nullptr) {}
+  MessagePortChild();
 
   void SetPort(MessagePort* aPort)
   {
     mPort = aPort;
   }
 
 private:
   ~MessagePortChild()