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

mojo/edk/util/weak_ptr_internal.cc

+// 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.
+
+#include "mojo/edk/util/weak_ptr_internal.h"
+
+#include <assert.h>
+
+namespace mojo {
+namespace util {
+namespace internal {
+
+WeakPtrFlag::WeakPtrFlag() : is_valid_(true) {}
+
+WeakPtrFlag::~WeakPtrFlag() {
+  // Should be invalidated before destruction.
+  assert(!is_valid_);
+}
+
+void WeakPtrFlag::Invalidate() {
+  // Invalidation should happen exactly once.
+  assert(is_valid_);
+  is_valid_ = false;
+}
+
+}  // namespace internal
+}  // namespace util
+}  // namespace mojo