Fix for 3303 MultithreadedParallelIsolates has a race condition.

src/codegen.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_CODEGEN_H_
 #define V8_CODEGEN_H_
 
 #include "code-stubs.h"
 #include "runtime.h"
 
@@ skipping 102 matching lines @@
                                      AllocationSiteMode mode,
                                      Label* fail);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ElementsTransitionGenerator);
 };
 
 static const int kNumberDictionaryProbes = 4;
 
 
+class CodeAgingHelper {
+ public:
+  CodeAgingHelper();
+
+  uint32_t young_sequence_length() const { return young_sequence_.length(); }
+  bool IsYoung(byte* candidate) const {
+    return memcmp(candidate,
+                  young_sequence_.start(),
+                  young_sequence_.length()) == 0;
+  }
+  void CopyYoungSequenceTo(byte* new_buffer) const {
+    CopyBytes(new_buffer, young_sequence_.start(), young_sequence_.length());
+  }
+
+#ifdef DEBUG
+  bool IsOld(byte* candidate) const;
+#endif
+
+ protected:
+  const EmbeddedVector<byte, kNoCodeAgeSequenceLength> young_sequence_;
+#ifdef DEBUG
+#ifdef V8_TARGET_ARCH_ARM64
+  const EmbeddedVector<byte, kNoCodeAgeSequenceLength> old_sequence_;
+#endif
+#endif
+};
+
 } }  // namespace v8::internal
 
 #endif  // V8_CODEGEN_H_