Replace __init__ function in asm-wasm-builder with the start function section

src/wasm/encoder.h

 // Copyright 2015 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_WASM_ENCODER_H_
 #define V8_WASM_ENCODER_H_
 
 #include "src/signature.h"
 #include "src/zone-containers.h"
 
@@ skipping 102 matching lines @@
   WasmModuleIndex* WriteTo(Zone* zone) const;
 
  private:
   friend class WasmModuleBuilder;
   explicit WasmModuleWriter(Zone* zone);
   ZoneVector<WasmFunctionEncoder*> functions_;
   ZoneVector<WasmDataSegmentEncoder*> data_segments_;
   ZoneVector<FunctionSig*> signatures_;
   ZoneVector<uint16_t> indirect_functions_;
   ZoneVector<std::pair<MachineType, bool>> globals_;
+  int start_function_index_;
 };
 
 class WasmModuleBuilder : public ZoneObject {
  public:
   explicit WasmModuleBuilder(Zone* zone);
   uint16_t AddFunction();
   uint32_t AddGlobal(MachineType type, bool exported);
   WasmFunctionBuilder* FunctionAt(size_t index);
   void AddDataSegment(WasmDataSegmentEncoder* data);
   uint16_t AddSignature(FunctionSig* sig);
   void AddIndirectFunction(uint16_t index);
+  void MarkStartFunction(uint16_t index);
   WasmModuleWriter* Build(Zone* zone);
 
   struct CompareFunctionSigs {
     bool operator()(FunctionSig* a, FunctionSig* b) const;
   };
   typedef ZoneMap<FunctionSig*, uint16_t, CompareFunctionSigs> SignatureMap;
 
  private:
   Zone* zone_;
   ZoneVector<FunctionSig*> signatures_;
   ZoneVector<WasmFunctionBuilder*> functions_;
   ZoneVector<WasmDataSegmentEncoder*> data_segments_;
   ZoneVector<uint16_t> indirect_functions_;
   ZoneVector<std::pair<MachineType, bool>> globals_;
   SignatureMap signature_map_;
+  int start_function_index_;
 };
 
 std::vector<uint8_t> UnsignedLEB128From(uint32_t result);
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_WASM_ENCODER_H_