First version of PerformanceObserver

Source/core/timing/PerformanceObserver.idl

+// 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/#idl-def-PerformanceObserverCallback
+callback PerformanceObserverCallback = void (PerformanceObserverEntryList entries, PerformanceObserver observer);
+
+// http://w3c.github.io/performance-timeline/#the-performance-observer-interface
+[
+    CustomConstructor(PerformanceObserverCallback callback),
+    RuntimeEnabled=PerformanceObserver,
+    GarbageCollected,
+] interface PerformanceObserver {
+    [RaisesException] void observe(PerformanceObserverInit options);
+    void disconnect();
+};