Add forward_variables_from() and target() to GN

tools/gn/scope.cc

Index: tools/gn/scope.cc
diff --git a/tools/gn/scope.cc b/tools/gn/scope.cc
index bee4ce0aa2dedc766c6fe7b34faea55fd0c1554d..d1f179d98106869e56235db772df8f33d869c74f 100644
--- a/tools/gn/scope.cc
+++ b/tools/gn/scope.cc
@@ -116,6 +116,17 @@ Value* Scope::GetValueForcedToCurrentScope(const base::StringPiece& ident,
   return nullptr;
 }
 
+base::StringPiece Scope::GetStorageKey(const base::StringPiece& ident) const {
+  RecordMap::const_iterator found = values_.find(ident);
+  if (found != values_.end())
+    return found->first;
+
+  // Search in parent scope.
+  if (containing())
+    return containing()->GetStorageKey(ident);
+  return base::StringPiece();
+}
+
 const Value* Scope::GetValue(const base::StringPiece& ident) const {
   RecordMap::const_iterator found = values_.find(ident);
   if (found != values_.end())
@@ -180,6 +191,11 @@ void Scope::MarkUsed(const base::StringPiece& ident) {
   found->second.used = true;
 }
 
+void Scope::MarkAllUsed() {
+  for (auto& cur : values_)
+    cur.second.used = true;
+}
+
 void Scope::MarkUnused(const base::StringPiece& ident) {
   RecordMap::iterator found = values_.find(ident);
   if (found == values_.end()) {
@@ -255,7 +271,7 @@ bool Scope::NonRecursiveMergeTo(Scope* dest,
     }
     dest->values_[pair.first] = pair.second;
 
-    if (options.mark_used)
+    if (options.mark_dest_used)
       dest->MarkUsed(pair.first);
   }