Mojo: Cancel WaitingCallbacks when their HandleWrappers are closed.

mojo/bindings/js/handle.h

Index: mojo/bindings/js/handle.h
diff --git a/mojo/bindings/js/handle.h b/mojo/bindings/js/handle.h
index 35202b07ea674240bdc4b81c6702f718f8b9d696..38754e7c7e156fd08ecfe853e98d9fd940829cf7 100644
--- a/mojo/bindings/js/handle.h
+++ b/mojo/bindings/js/handle.h
@@ -5,6 +5,8 @@
 #ifndef MOJO_BINDINGS_JS_HANDLE_H_
 #define MOJO_BINDINGS_JS_HANDLE_H_
 
+#include <set>
+
 #include "gin/converter.h"
 #include "gin/handle.h"
 #include "gin/wrappable.h"
@@ -16,6 +18,11 @@ namespace gin {
 // is Closed when its JS object is garbage collected.
 class HandleWrapper : public gin::Wrappable<HandleWrapper> {
  public:
+  class CloseObserver {
+   public:
+    virtual ~CloseObserver() {}
+    virtual void OnHandleClosed() = 0;
+  };
   static gin::WrapperInfo kWrapperInfo;
 
   static gin::Handle<HandleWrapper> Create(v8::Isolate* isolate,
@@ -25,12 +32,17 @@ class HandleWrapper : public gin::Wrappable<HandleWrapper> {
 
   mojo::Handle get() const { return handle_.get(); }
   mojo::Handle release() { return handle_.release(); }
-  void Close() { handle_.reset(); }
+  void Close();
+
+  void AddCloseObserver(CloseObserver* observer);
+  void RemoveCloseObserver(CloseObserver* observer);
 
  protected:
   HandleWrapper(MojoHandle handle);
   virtual ~HandleWrapper();
+  void NotifyCloseObservers();
   mojo::ScopedHandle handle_;
+  std::set<CloseObserver*> observers_;
 };
 
 // Note: It's important to use this converter rather than the one for