base: Add SetEnv() to EnvVarGetter class and get rid of the some ifdefs.

base/env_var.h

 // Copyright (c) 2010 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 BASE_ENV_VAR_H_
 #define BASE_ENV_VAR_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 
 namespace base {
 
 // These are used to derive mocks for unittests.
 class EnvVarGetter {
  public:
   virtual ~EnvVarGetter() {}
   // Gets an environment variable's value and stores it in |result|.
   // Returns false if the key is unset.
   virtual bool GetEnv(const char* variable_name, std::string* result) = 0;
 
   // Syntactic sugar for GetEnv(variable_name, NULL);
   virtual bool HasEnv(const char* variable_name) {
     return GetEnv(variable_name, NULL);
   }
 
+  virtual void SetEnv(const char* variable_name,
+                      const std::string& new_value) = 0;
+
   // Create an instance of EnvVarGetter
   static EnvVarGetter* Create();
 };
 
 }  // namespace base
 
 #endif  // BASE_ENV_VAR_H_