Add forward_variables_from() and target() to GN

tools/gn/test_with_scope.h

Index: tools/gn/test_with_scope.h
diff --git a/tools/gn/test_with_scope.h b/tools/gn/test_with_scope.h
index e390df84a59735d9433e8983d9b52a6a575fafad..ba9515d56022cc45e563a28cef04848c7b982e6c 100644
--- a/tools/gn/test_with_scope.h
+++ b/tools/gn/test_with_scope.h
@@ -13,6 +13,7 @@
 #include "tools/gn/input_file.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
+#include "tools/gn/scope_per_file_provider.h"
 #include "tools/gn/settings.h"
 #include "tools/gn/target.h"
 #include "tools/gn/token.h"
@@ -59,6 +60,9 @@ class TestWithScope {
   Toolchain toolchain_;
   Scope scope_;
 
+  // Supplies the scope with built-in variables like root_out_dir.
+  ScopePerFileProvider scope_progammatic_provider_;
+
   std::string print_output_;
 
   DISALLOW_COPY_AND_ASSIGN(TestWithScope);