Mojo: Cancel WaitingCallbacks when their HandleWrappers are closed.

mojo/bindings/js/handle_close_observer.h

Index: mojo/bindings/js/handle_close_observer.h
diff --git a/mojo/bindings/js/handle_close_observer.h b/mojo/bindings/js/handle_close_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..854603ef5d9203dcfb1be496f8a1543dd3050b4e
--- /dev/null
+++ b/mojo/bindings/js/handle_close_observer.h
@@ -0,0 +1,20 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MOJO_BINDINGS_JS_HANDLE_CLOSE_OBSERVER_H_
+#define MOJO_BINDINGS_JS_HANDLE_CLOSE_OBSERVER_H_
+
+namespace gin {
+
+class HandleCloseObserver {
+ public:
+  virtual void OnWillCloseHandle() = 0;
+
+ protected:
+  virtual ~HandleCloseObserver() {}
+};
+
+}  // namespace gin
+
+#endif  // MOJO_BINDINGS_JS_HANDLE_CLOSE_OBSERVER_H_