[wasm] Cleanup AST decoder. Remove Tree and TreeResult.

src/wasm/module-decoder.cc

Index: src/wasm/module-decoder.cc
diff --git a/src/wasm/module-decoder.cc b/src/wasm/module-decoder.cc
index d99029517e0584752a155c54bddfbe4800bd7ce6..435ded6b1a807dd5c6cc34d486f3df3ba9f0e7bc 100644
--- a/src/wasm/module-decoder.cc
+++ b/src/wasm/module-decoder.cc
@@ -546,7 +546,7 @@ class ModuleDecoder : public Decoder {
     FunctionBody body = {menv, function->sig, start_,
                          start_ + function->code_start_offset,
                          start_ + function->code_end_offset};
-    TreeResult result = VerifyWasmCode(module_zone->allocator(), body);
+    DecodeResult result = VerifyWasmCode(module_zone->allocator(), body);
     if (result.failed()) {
       // Wrap the error message from the function decoder.
       std::ostringstream str;