Remove default value checking in GN, adds getenv function, reorders parameters to rebase_path.

tools/gn/functions.h

 // Copyright (c) 2013 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 TOOLS_GN_FUNCTIONS_H_
 #define TOOLS_GN_FUNCTIONS_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 103 matching lines @@
                     Err* err);
 
 extern const char kExecutable[];
 extern const char kExecutable_Help[];
 Value RunExecutable(Scope* scope,
                     const FunctionCallNode* function,
                     const std::vector<Value>& args,
                     BlockNode* block,
                     Err* err);
 
+extern const char kGetEnv[];
+extern const char kGetEnv_Help[];
+Value RunGetEnv(Scope* scope,
+                const FunctionCallNode* function,
+                const std::vector<Value>& args,
+                Err* err);
+
 extern const char kGroup[];
 extern const char kGroup_Help[];
 Value RunGroup(Scope* scope,
                const FunctionCallNode* function,
                const std::vector<Value>& args,
                BlockNode* block,
                Err* err);
 
 extern const char kImport[];
 extern const char kImport_Help[];
@@ skipping 200 matching lines @@
 // Returns the name of the toolchain for the given scope.
 const Label& ToolchainLabelForScope(const Scope* scope);
 
 // Generates a label for the given scope, using the current directory and
 // toolchain, and the given name.
 Label MakeLabelForScope(const Scope* scope,
                         const FunctionCallNode* function,
                         const std::string& name);
 
 #endif  // TOOLS_GN_FUNCTIONS_H_