Add an experimental public API for value serialization.

src/value-serializer.h

Index: src/value-serializer.h
diff --git a/src/value-serializer.h b/src/value-serializer.h
index ab9c66489963569e8d53f5fcca31b013bdf59561..c94f883a825fea752f4d712c3bf235beb1009e18 100644
--- a/src/value-serializer.h
+++ b/src/value-serializer.h
@@ -116,6 +116,13 @@ class ValueDeserializer {
   Maybe<bool> ReadHeader() WARN_UNUSED_RESULT;
 
   /*
+   * Reads the underlying wire format version. Likely mostly to be useful to
+   * legacy code reading old wire format versions. Must be called after
+   * ReadHeader.
+   */
+  uint32_t GetWireFormatVersion() const { return version_; }
+
+  /*
    * Deserializes a V8 object from the buffer.
    */
   MaybeHandle<Object> ReadObject() WARN_UNUSED_RESULT;