Adds a way to associate key/value pairs with the environment

mojo/public/cpp/environment/environment.h

 // 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_PUBLIC_CPP_ENVIRONMENT_ENVIRONMENT_H_
 #define MOJO_PUBLIC_CPP_ENVIRONMENT_ENVIRONMENT_H_
 
 #include "mojo/public/cpp/system/macros.h"
 
 namespace mojo {
 
 // This class must be instantiated before using any Mojo APIs.
 class Environment {
  public:
   Environment();
   ~Environment();
 
  private:
+  class Data;
+
+  // Environment implementation can use this to store state.
+  Data* data_;
+
   MOJO_DISALLOW_COPY_AND_ASSIGN(Environment);
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_ENVIRONMENT_ENVIRONMENT_H_