[wasm] Rename _WASMEXP_ object to Wasm.

test/mjsunit/wasm/verify-function-simple.js

 // 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.
 
 // Flags: --expose-wasm
 
 load("test/mjsunit/wasm/wasm-constants.js");
 
 try {
   var data = bytes(
       0,       kAstStmt,  // signature
       kDeclNoLocals,      // --
       kExprNop            // body
   );
 
-  _WASMEXP_.verifyFunction(data);
+  Wasm.verifyFunction(data);
   print("ok");
 } catch (e) {
   assertTrue(false);
 }
 
 
 var threw = false;
 try {
   var data = bytes(
       0,       kAstI32,   // signature
       kDeclNoLocals,      // --
       kExprBlock, 2, kExprNop, kExprNop  // body
   );
 
-  _WASMEXP_.verifyFunction(data);
+  Wasm.verifyFunction(data);
   print("not ok");
 } catch (e) {
   print("ok: " + e);
   threw = true;
 }
 
 assertTrue(threw);