[wasm] Refactor encoder.h to use a proper buffer and remove OldFunctions section.

src/wasm/asm-wasm-builder.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_ASM_WASM_BUILDER_H_
 #define V8_WASM_ASM_WASM_BUILDER_H_
 
 #include "src/allocation.h"
 #include "src/objects.h"
 #include "src/typing-asm.h"
 #include "src/wasm/encoder.h"
 #include "src/zone.h"
 
 namespace v8 {
 namespace internal {
 
 class FunctionLiteral;
 
 namespace wasm {
 
 class AsmWasmBuilder {
  public:
   explicit AsmWasmBuilder(Isolate* isolate, Zone* zone, FunctionLiteral* root,
                           AsmTyper* typer);
-  WasmModuleIndex* Run(Handle<FixedArray>* foreign_args);
+  ZoneBuffer* Run(Handle<FixedArray>* foreign_args);
 
  private:
   Isolate* isolate_;
   Zone* zone_;
   FunctionLiteral* literal_;
   AsmTyper* typer_;
 };
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_WASM_ASM_WASM_BUILDER_H_