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

test/mjsunit/wasm/import-memory.js

 // 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
 
 load("test/mjsunit/wasm/wasm-constants.js");
 load("test/mjsunit/wasm/wasm-module-builder.js");
 
 (function TestOne() {
@@ skipping 348 matching lines @@
       assertEquals(expected_pages, instances[i].exports.mem_size());
     }
   }
   var current_mem_size = 1;
   for (var i = 0; i < 10; i++) {
     function grow(pages) { return instances[i].exports.grow(pages); }
     assertEquals(current_mem_size, instances[i].exports.grow(1));
     verify_mem_size(++current_mem_size);
   }
 })();
+
+(function TestExportGrow() {
+  print("TestExportGrow");
+  let builder = new WasmModuleBuilder();
+  builder.addMemory(1, 5, true);
+  builder.exportMemoryAs("exported_mem");
+  builder.addFunction("mem_size", kSig_i_v)
+    .addBody([kExprMemorySize, kMemoryZero])
+    .exportFunc();
+  builder.addFunction("grow", kSig_i_i)
+    .addBody([kExprGetLocal, 0, kExprGrowMemory, kMemoryZero])
+    .exportFunc();
+  instance = builder.instantiate();
+  assertEquals(kPageSize, instance.exports.exported_mem.buffer.byteLength);
+  assertEquals(1, instance.exports.grow(2));
+  assertEquals(3, instance.exports.mem_size());
+  assertEquals(3*kPageSize, instance.exports.exported_mem.buffer.byteLength);
+})();