[ic] Ensure prototype validity cell guards global object's prototype changes for LoadGlobalIC.

BUG=chromium:666742, v8:5561

Review-Url: https://codereview.chromium.org/2512183002
Cr-Commit-Position: refs/heads/master@{#41136}

src/objects.cc

@@ -12773,9 +12773,17 @@ void Map::SetShouldBeFastPrototypeMap(Handle<Map> map, bool value,
 // static
 Handle<Cell> Map::GetOrCreatePrototypeChainValidityCell(Handle<Map> map,
                                                         Isolate* isolate) {
-  Handle<Object> maybe_prototype(
-      map->GetPrototypeChainRootMap(isolate)->prototype(), isolate);
+  Handle<Object> maybe_prototype;
+  if (map->IsJSGlobalObjectMap()) {
+    DCHECK(map->is_prototype_map());
+    // Global object is prototype of a global proxy and therefore we can
+    // use its validity cell for guarding global object's prototype change.
+    maybe_prototype = isolate->global_object();
+  } else {
+    maybe_prototype =
+        handle(map->GetPrototypeChainRootMap(isolate)->prototype(), isolate);
     if (!maybe_prototype->IsJSObject()) return Handle<Cell>::null();
+  }
   Handle<JSObject> prototype = Handle<JSObject>::cast(maybe_prototype);
   // Ensure the prototype is registered with its own prototypes so its cell
   // will be invalidated when necessary.

test/mjsunit/regress/regress-crbug-666742.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-gc
+
+var p = {x:1};
+__proto__ = p;
+assertEquals(x, 1);
+__proto__ = {x:13};
+assertEquals(x, 13);
+__proto__ = {x:42};
+p = null;
+gc();
+assertEquals(x, 42);