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

src/wasm/asm-wasm-builder.cc

 // 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.
 
 #include "src/v8.h"
 
 // Required to get M_E etc. in MSVC.
 #if defined(_WIN32)
 #define _USE_MATH_DEFINES
 #endif
@@ skipping 1741 matching lines @@
  private:
   DISALLOW_COPY_AND_ASSIGN(AsmWasmBuilderImpl);
 };
 
 AsmWasmBuilder::AsmWasmBuilder(Isolate* isolate, Zone* zone,
                                FunctionLiteral* literal, AsmTyper* typer)
     : isolate_(isolate), zone_(zone), literal_(literal), typer_(typer) {}
 
 // TODO(aseemgarg): probably should take zone (to write wasm to) as input so
 // that zone in constructor may be thrown away once wasm module is written.
-WasmModuleIndex* AsmWasmBuilder::Run(i::Handle<i::FixedArray>* foreign_args) {
+ZoneBuffer* AsmWasmBuilder::Run(i::Handle<i::FixedArray>* foreign_args) {
   AsmWasmBuilderImpl impl(isolate_, zone_, literal_, typer_);
   impl.Compile();
   *foreign_args = impl.GetForeignArgs();
+  ZoneBuffer* buffer = new (zone_) ZoneBuffer(zone_);
   WasmModuleWriter* writer = impl.builder_->Build(zone_);
-  return writer->WriteTo(zone_);
+  writer->WriteTo(*buffer);
+  return buffer;
 }
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8