Do not save/restore AST id generator.

src/preparser.cc

Index: src/preparser.cc
diff --git a/src/preparser.cc b/src/preparser.cc
index 3173cc0f900255aa1d764736d8c6f70d51da0ddb..27a7b9ed4bc706a04e7bed2e9b2e90bf8448924f 100644
--- a/src/preparser.cc
+++ b/src/preparser.cc
@@ -32,13 +32,6 @@ int isfinite(double value);
 namespace v8 {
 namespace internal {
 
-class PreParserTraits::Checkpoint
-    : public ParserBase<PreParserTraits>::CheckpointBase {
- public:
-  explicit Checkpoint(ParserBase<PreParserTraits>* parser)
-      : ParserBase<PreParserTraits>::CheckpointBase(parser) {}
-};
-
 void PreParserTraits::ReportMessageAt(Scanner::Location location,
                                       const char* message,
                                       const char* arg,