Cleanup: Pass std::string as const reference if possible

tools/gn/value.h

Index: tools/gn/value.h
diff --git a/tools/gn/value.h b/tools/gn/value.h
index 44fba4adb2af16a450eda4608a767ed178a2bb1e..6556f8d6d9a73cda76260cc322e54f1e71876fcc 100644
--- a/tools/gn/value.h
+++ b/tools/gn/value.h
@@ -32,7 +32,8 @@ class Value {
   Value(const ParseNode* origin, Type t);
   Value(const ParseNode* origin, bool bool_val);
   Value(const ParseNode* origin, int64_t int_val);
-  Value(const ParseNode* origin, std::string str_val);
+  Value(const ParseNode* origin, const std::string& str_val);
+  Value(const ParseNode* origin, std::string&& str_val) noexcept;
   Value(const ParseNode* origin, const char* str_val);
   // Values "shouldn't" have null scopes when type == Scope, so be sure to
   // always set one. However, this is not asserted since there are some