Reland r54418 - base: Add UnSetEnv function to EnvVarGetter API.

base/env_var.h

Index: base/env_var.h
diff --git a/base/env_var.h b/base/env_var.h
index 9622e74768474a5353b9cf021bfc695d01d44d17..43d3fd64d06a90926064eb8494da644af80f3e84 100644
--- a/base/env_var.h
+++ b/base/env_var.h
@@ -20,11 +20,14 @@ extern const char kHome[];
 
 }  // namespace env_vars
 
-// These are used to derive mocks for unittests.
 class EnvVarGetter {
  public:
   virtual ~EnvVarGetter();
 
+  // Static factory method that returns the implementation that provide the
+  // appropriate platform-specific instance.
+  static EnvVarGetter* Create();
+
   // 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;
@@ -36,8 +39,8 @@ class EnvVarGetter {
   virtual bool SetEnv(const char* variable_name,
                       const std::string& new_value) = 0;
 
-  // Create an instance of EnvVarGetter
-  static EnvVarGetter* Create();
+  // Returns true on success, otherwise returns false.
+  virtual bool UnSetEnv(const char* variable_name) = 0;
 };
 
 }  // namespace base