[turbofan] Add support for advanced reducers.

test/unittests/compiler/graph-reducer-unittest.h

+// Copyright 2015 the V8 project 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 V8_UNITTESTS_COMPILER_GRAPH_REDUCER_UNITTEST_H_
+#define V8_UNITTESTS_COMPILER_GRAPH_REDUCER_UNITTEST_H_
+
+#include "src/compiler/graph-reducer.h"
+#include "testing/gmock/include/gmock/gmock.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+struct MockReducerObserver : public Reducer::Observer {
+  MOCK_METHOD1(Revisit, void(Node*));
+  MOCK_METHOD2(Replace, void(Node*, Node*));
+};
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_UNITTESTS_COMPILER_GRAPH_REDUCER_UNITTEST_H_