Add forward_variables_from() and target() to GN

tools/gn/import_manager.cc

Index: tools/gn/import_manager.cc
diff --git a/tools/gn/import_manager.cc b/tools/gn/import_manager.cc
index 9c08d12f15f7956f87be41022d30e7bd62857fba..2d39846d981b89d99efa50eb8989cfd14b17f676 100644
--- a/tools/gn/import_manager.cc
+++ b/tools/gn/import_manager.cc
@@ -86,7 +86,7 @@ bool ImportManager::DoImport(const SourceFile& file,
 
   Scope::MergeOptions options;
   options.skip_private_vars = true;
-  options.mark_used = true;  // Don't require all imported values be used.
+  options.mark_dest_used = true;  // Don't require all imported values be used.
   return imported_scope->NonRecursiveMergeTo(scope, options, node_for_err,
                                              "import", err);
 }