Decouple performance() from LocalDOMWindow

Source/core/timing/WindowPerformance.idl

Index: Source/core/timing/WindowPerformance.idl
diff --git a/Source/core/timing/WindowPerformance.idl b/Source/core/timing/WindowPerformance.idl
new file mode 100644
index 0000000000000000000000000000000000000000..bc38d37c8dfa0fe786c23770e8b76d7f528f7f1c
--- /dev/null
+++ b/Source/core/timing/WindowPerformance.idl
@@ -0,0 +1,10 @@
+// Copyright 2015 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.
+
+[
+    ImplementedAs=DOMWindowPerformance,
+] partial interface Window {
+    // https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/NavigationTiming/Overview.html#sec-window.performance-attribute
+    [Replaceable] readonly attribute Performance performance;
+};