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

src/wasm/asm-wasm-builder.h

Index: src/wasm/asm-wasm-builder.h
diff --git a/src/wasm/asm-wasm-builder.h b/src/wasm/asm-wasm-builder.h
index 3dfda5ef1ae740131ccedd93add2c2e2cfb5c06a..b99c3effa1e85b3b8ea29185ca31ba3ac379d5cc 100644
--- a/src/wasm/asm-wasm-builder.h
+++ b/src/wasm/asm-wasm-builder.h
@@ -22,7 +22,7 @@ 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_;