Support for excluding variable from forwarding via forward_variables_form.

tools/gn/scope.h

Index: tools/gn/scope.h
diff --git a/tools/gn/scope.h b/tools/gn/scope.h
index 7b329412450ab24ecdf7369faa58c33c41ed84d6..2554366a90a37e39876cc9895b8bffb99fa24454 100644
--- a/tools/gn/scope.h
+++ b/tools/gn/scope.h
@@ -65,12 +65,8 @@ class Scope {
 
   // Options for configuring scope merges.
   struct MergeOptions {
-    // Defaults to all false, which are the things least likely to cause errors.
-    MergeOptions()
-        : clobber_existing(false),
-          skip_private_vars(false),
-          mark_dest_used(false) {
-    }
+    MergeOptions();
+    ~MergeOptions();
 
     // When set, all existing avlues in the destination scope will be
     // overwritten.
@@ -92,6 +88,9 @@ class Scope {
     // import, for example, or files that don't need a variable from the .gni
     // file will throw an error.
     bool mark_dest_used;
+
+    // When set, those variables are not merged.
+    std::set<std::string> excluded_values;
   };
 
   // Creates an empty toplevel scope.