[wasm] Update WasmMemoryObject correctly when module memory is exported.

test/mjsunit/regress/wasm/regression-670683.js

Index: test/mjsunit/regress/wasm/regression-670683.js
diff --git a/test/mjsunit/regress/wasm/regression-670683.js b/test/mjsunit/regress/wasm/regression-670683.js
new file mode 100644
index 0000000000000000000000000000000000000000..68a890ce2ef1a1e0d67c286505277954442f541b
--- /dev/null
+++ b/test/mjsunit/regress/wasm/regression-670683.js
@@ -0,0 +1,17 @@
+// Copyright 2016 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
+
+const bytes = new Uint8Array([
+  0x00, 0x61, 0x73, 0x6d, 0x0d, 0x00, 0x00, 0x00,
+  0x01, 0x05, 0x01, 0x60, 0x00, 0x01, 0x7f, 0x03,
+  0x02, 0x01, 0x00, 0x05, 0x03, 0x01, 0x00, 0x01,
+  0x07, 0x11, 0x02, 0x04, 0x67, 0x72, 0x6f, 0x77,
+  0x00, 0x00, 0x06, 0x6d, 0x65, 0x6d, 0x6f, 0x72,
+  0x79, 0x02, 0x00, 0x0a, 0x07, 0x01, 0x05, 0x00,
+  0x41, 0x01, 0x40, 0x00]);
+
+WebAssembly.compile(bytes).then(
+    m => new WebAssembly.Instance(m).exports.grow());