[DOM] Make IORunnable::mFilePickerParent into a RefPtr.

dom/ipc/FilePickerParent.cpp

@@ -286,6 +286,7 @@ FilePickerParent::RecvOpen(const int16_t& aSelectedType,
     }
   }
 
+  MOZ_ASSERT(!mCallback);
   mCallback = new FilePickerShownCallback(this);
 
   mFilePicker->Open(mCallback);

dom/ipc/FilePickerParent.h

@@ -69,7 +69,7 @@ class FilePickerParent : public PFilePickerParent
 
   private:
     virtual ~FilePickerShownCallback() {}
-    FilePickerParent* mFilePickerParent;
+    RefPtr<FilePickerParent> mFilePickerParent;
   };
 
  private:
@@ -78,7 +78,7 @@ class FilePickerParent : public PFilePickerParent
   // This runnable is used to do some I/O operation on a separate thread.
   class IORunnable : public Runnable
   {
-    FilePickerParent* mFilePickerParent;
+    RefPtr<FilePickerParent> mFilePickerParent;
     nsTArray<nsCOMPtr<nsIFile>> mFiles;
     nsTArray<BlobImplOrString> mResults;
     nsCOMPtr<nsIEventTarget> mEventTarget;