Created placeholders for MallocHooks implementation and added define DART_USE_TCMALLOC to specifiy …

runtime/vm/malloc_hooks.h

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#ifndef RUNTIME_VM_MALLOC_HOOKS_H_
+#define RUNTIME_VM_MALLOC_HOOKS_H_
+
+#include "vm/globals.h"
+
+namespace dart {
+
+class MallocHooks {
+  static void Init();
+
+ private:
+  DISALLOW_ALLOCATION();
+  DISALLOW_IMPLICIT_CONSTRUCTORS(MallocHooks);
+};
+
+}  // namespace dart
+
+#endif  // RUNTIME_VM_MALLOC_HOOKS_H_