Bring in a reduced/simplified version of Chromium's base::WeakPtr to //mojo/edk/util.

mojo/edk/util/weak_ptr_internal.h

+// Copyright 2016 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_UTIL_WEAK_PTR_INTERNAL_H_
+#define MOJO_UTIL_WEAK_PTR_INTERNAL_H_
+
+#include <assert.h>
+
+#include "mojo/edk/util/ref_counted.h"
+#include "mojo/public/cpp/system/macros.h"
+
+namespace mojo {
+namespace util {
+namespace internal {
+
+// |WeakPtr<T>|s have a reference to a |WeakPtrFlag| to determine whether they
+// are valid (non-null) or not. We do not store a |T*| in this object since
+// there may also be |WeakPtr<U>|s to the same object, where |U| is a superclass
+// of |T|.
+//
+// This class in not thread-safe, though references may be released on any
+// thread (allowing weak pointers to be destroyed/reset/reassigned on any
+// thread).
+class WeakPtrFlag : public RefCountedThreadSafe<WeakPtrFlag> {
+ public:
+  WeakPtrFlag();
+  ~WeakPtrFlag();
+
+  bool is_valid() const { return is_valid_; }
+
+  void Invalidate();
+
+ private:
+  bool is_valid_;
+
+  MOJO_DISALLOW_COPY_AND_ASSIGN(WeakPtrFlag);
+};
+
+}  // namespace internal
+}  // namespace util
+}  // namespace mojo
+
+#endif  // MOJO_UTIL_WEAK_PTR_INTERNAL_H_