Fix cluster-fuzz bug introduced in refs/heads/master@{#28796}.

Don't DCHECK when neutering that the buffer is not a SharedArrayBuffer;
instead, just return early.

BUG=chromium:498142,chromium:497295
R=jarin@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1174753002

Cr-Commit-Position: refs/heads/master@{#28892}

src/runtime/runtime-typedarray.cc

@@ -144,7 +144,7 @@ RUNTIME_FUNCTION(Runtime_ArrayBufferNeuter) {
     return isolate->heap()->undefined_value();
   }
   // Shared array buffers should never be neutered.
-  DCHECK(!array_buffer->is_shared());
+  RUNTIME_ASSERT(!array_buffer->is_shared());
   DCHECK(!array_buffer->is_external());
   void* backing_store = array_buffer->backing_store();
   size_t byte_length = NumberToSize(isolate, array_buffer->byte_length());

test/mjsunit/regress/regress-crbug-498142.js

+// Copyright 2015 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: --allow-natives-syntax --harmony-sharedarraybuffer
+
+var sab = new SharedArrayBuffer(16);
+assertThrows(function() { %ArrayBufferNeuter(sab); });