[DOM] Stop speech synthesis if the originating document is closed.

dom/media/webspeech/synth/SpeechSynthesis.cpp

@@ -293,6 +293,12 @@ SpeechSynthesis::ForceEnd()
   }
 }
 
+void SpeechSynthesis::DisconnectFromOwner() {
+  Pause();
+  Cancel();
+  DOMEventTargetHelper::DisconnectFromOwner();
+}
+
 NS_IMETHODIMP
 SpeechSynthesis::Observe(nsISupports* aSubject, const char* aTopic,
                          const char16_t* aData)

dom/media/webspeech/synth/SpeechSynthesis.h

@@ -58,6 +58,8 @@ public:
   void GetVoices(nsTArray< RefPtr<SpeechSynthesisVoice> >& aResult);
 
   void ForceEnd();
+  
+  void DisconnectFromOwner() override;
 
   IMPL_EVENT_HANDLER(voiceschanged)