Support for excluding variable from forwarding via forward_variables_form.

tools/gn/scope.cc

Index: tools/gn/scope.cc
diff --git a/tools/gn/scope.cc b/tools/gn/scope.cc
index 2bf9dd467da7903b5ff4a874915d5f1481429fea..ee2b1fe4d2bda1e3dedc5fda63f1e9d630d7e2c4 100644
--- a/tools/gn/scope.cc
+++ b/tools/gn/scope.cc
@@ -25,6 +25,13 @@ bool IsPrivateVar(const base::StringPiece& name) {
 
 }  // namespace
 
+// Defaults to all false, which are the things least likely to cause errors.
+Scope::MergeOptions::MergeOptions()
+    : clobber_existing(false),
+      skip_private_vars(false),
+      mark_dest_used(false) {}
+
+Scope::MergeOptions::~MergeOptions() {}
 
 Scope::ProgrammaticProvider::~ProgrammaticProvider() {
   scope_->RemoveProvider(this);
@@ -251,6 +258,11 @@ bool Scope::NonRecursiveMergeTo(Scope* dest,
   for (const auto& pair : values_) {
     if (options.skip_private_vars && IsPrivateVar(pair.first))
       continue;  // Skip this private var.
+    if (!options.excluded_values.empty() &&
+        options.excluded_values.find(pair.first.as_string()) !=
+            options.excluded_values.end()) {
+      continue;  // Skip this excluded value.
+    }
 
     const Value& new_value = pair.second.value;
     if (!options.clobber_existing) {