GN: Mark variables in nested scopes used for += and -=.

tools/gn/operators.cc

Index: tools/gn/operators.cc
diff --git a/tools/gn/operators.cc b/tools/gn/operators.cc
index 9ac18bfeff0b00cbef35f2e31bf82974927b8638..72b2dd18ce63ade447721cbc8a199c5b24699b9b 100644
--- a/tools/gn/operators.cc
+++ b/tools/gn/operators.cc
@@ -155,7 +155,7 @@ bool ValueDestination::Init(Scope* exec_scope,
 
 const Value* ValueDestination::GetExistingValue() const {
   if (type_ == SCOPE)
-    return scope_->GetValue(name_token_->value(), false);
+    return scope_->GetValue(name_token_->value(), true);
   else if (type_ == LIST)
     return &list_->list_value()[index_];
   return nullptr;