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

test/unittests/wasm/ast-decoder-unittest.cc

Index: test/unittests/wasm/ast-decoder-unittest.cc
diff --git a/test/unittests/wasm/ast-decoder-unittest.cc b/test/unittests/wasm/ast-decoder-unittest.cc
index 8be0e414afed97a7683c0281c45ca7d609815d27..889cfb1b1a3ed1695f729675aafc22a5f5acd37f 100644
--- a/test/unittests/wasm/ast-decoder-unittest.cc
+++ b/test/unittests/wasm/ast-decoder-unittest.cc
@@ -84,7 +84,7 @@ class AstDecoderTest : public TestWithZone {
               const byte* end) {
     local_decls.Prepend(zone(), &start, &end);
     // Verify the code.
-    TreeResult result =
+    DecodeResult result =
         VerifyWasmCode(zone()->allocator(), module, sig, start, end);
 
     if (result.error_code != expected) {