Add DOMSharedArrayBuffer type

Source/core/dom/SharedArrayBuffer.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// https://www.khronos.org/registry/typedarray/specs/latest/#ARRAYBUFFER
+// Draft spec:
+// https://docs.google.com/document/d/1NDGA_gZJ7M7w1Bh8S0AoDyEqwDdRh4uSoTPSNn77PFk
 
 [
-    ImplementedAs=DOMArrayBuffer,
+    ImplementedAs=DOMSharedArrayBuffer,
     NoInterfaceObject,
-] interface ArrayBuffer {
+    RuntimeEnabled=SharedArrayBuffer,
+] interface SharedArrayBuffer {
     readonly attribute unsigned long byteLength;
 };