Fix WaitingCallback to not run JavaScript during GC

mojo/edk/js/waiting_callback.h

Index: mojo/edk/js/waiting_callback.h
diff --git a/mojo/edk/js/waiting_callback.h b/mojo/edk/js/waiting_callback.h
index 6b2ccc73dcadef40eeb613ececc396ed0577a243..5a9dfaffa884cbbb26f83c2a9bdb0cfe09b05edf 100644
--- a/mojo/edk/js/waiting_callback.h
+++ b/mojo/edk/js/waiting_callback.h
@@ -5,6 +5,7 @@
 #ifndef MOJO_EDK_JS_WAITING_CALLBACK_H_
 #define MOJO_EDK_JS_WAITING_CALLBACK_H_
 
+#include "base/memory/weak_ptr.h"
 #include "gin/handle.h"
 #include "gin/runner.h"
 #include "gin/wrappable.h"
@@ -49,10 +50,14 @@ class WaitingCallback : public gin::Wrappable<WaitingCallback>,
   // still in progress.
   void OnWillCloseHandle() override;
 
+  void ClearWaitId();
+  void CallCallback(MojoResult result);
+
   base::WeakPtr<gin::Runner> runner_;
   MojoAsyncWaitID wait_id_;
 
   HandleWrapper* handle_wrapper_;
+  base::WeakPtrFactory<WaitingCallback> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WaitingCallback);
 };