Bug 1137151: Marked destructor of |android::MediaCodecReader| as protected, r=sotaro

dom/media/omx/MediaCodecReader.h

@@ -275,10 +275,12 @@ private:
     NS_INLINE_DECL_THREADSAFE_REFCOUNTING(SignalObject)
 
     SignalObject(const char* aName);
-    ~SignalObject();
     void Wait();
     void Signal();
 
+  protected:
+    ~SignalObject();
+
   private:
     // Forbidden
     SignalObject() = delete;