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

test/mjsunit/wasm/import-memory.js

Index: test/mjsunit/wasm/import-memory.js
diff --git a/test/mjsunit/wasm/import-memory.js b/test/mjsunit/wasm/import-memory.js
index 3b70332f97bc5eb95d2dbd81135df3a89204d0c2..3f2cf1155a84991253eb957e7bc1c45fe3bd1ee8 100644
--- a/test/mjsunit/wasm/import-memory.js
+++ b/test/mjsunit/wasm/import-memory.js
@@ -366,3 +366,21 @@ load("test/mjsunit/wasm/wasm-module-builder.js");
     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);
+})();