[typedarray] Use native context in elements accessor.

A check will fail if the context passed in is not a native context.
Change the code to get the native context from the passed context.

Bug: chromium:804288
Change-Id: Iad314a3dd170355cf524b9230a692a6329564f8a
Reviewed-on: https://chromium-review.googlesource.com/878324Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50761}

src/elements.cc

@@ -3339,7 +3339,8 @@ class TypedElementsAccessor
     // them.
     if (source_proto->IsNull(isolate)) return false;
     if (source_proto->IsJSProxy()) return true;
-    if (!context->is_initial_array_prototype(JSObject::cast(source_proto))) {
+    if (!context->native_context()->is_initial_array_prototype(
+            JSObject::cast(source_proto))) {
       return true;
     }
 

test/mjsunit/regress/regress-804288.js

+// Copyright 2018 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.
+
+var arr = [{}];
+Object.setPrototypeOf(arr, {});
+var ta = new Uint8Array(arr);
+
+let kDeclNoLocals = 0;