[serializer] checksum a larger part of the code snapshot.

src/snapshot/code-serializer.h

Index: src/snapshot/code-serializer.h
diff --git a/src/snapshot/code-serializer.h b/src/snapshot/code-serializer.h
index 6a5937d72821f133cf72d77a3e98f693a20f2a6d..e82a7d5dd6d5787582263f850981e0988d3f8b22 100644
--- a/src/snapshot/code-serializer.h
+++ b/src/snapshot/code-serializer.h
@@ -115,6 +115,10 @@ class SerializedCodeData : public SerializedData {
   SerializedCodeData(const byte* data, int size)
       : SerializedData(const_cast<byte*>(data), size) {}
 
+  Vector<const byte> DataWithoutHeader() const {
+    return Vector<const byte>(data_ + kHeaderSize, size_ - kHeaderSize);
+  }
+
   SanityCheckResult SanityCheck(Isolate* isolate,
                                 uint32_t expected_source_hash) const;
   // The data header consists of uint32_t-sized entries: