First version of PerformanceObserver

Source/core/timing/PerformanceObserverInit.idl

Index: Source/core/timing/PerformanceObserverInit.idl
diff --git a/Source/core/timing/PerformanceObserverInit.idl b/Source/core/timing/PerformanceObserverInit.idl
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+++ b/Source/core/timing/PerformanceObserverInit.idl
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+// 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.
+
+// http://w3c.github.io/performance-timeline/#the-performance-observer-interface
+[
+    RuntimeEnabled=PerformanceObserverSupport,  // Experimental flag
+] dictionary PerformanceObserverInit {
+    required sequence<PerformanceEntryType> entryType;
+};