LongTaskTiming: add idl, Experiment flag, PerformanceEntry and basic implementation

third_party/WebKit/Source/core/timing/PerformanceLongTaskTiming.cpp

+// Copyright 2016 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.
+
+#include "core/timing/PerformanceLongTaskTiming.h"
+
+namespace blink {
+
+PerformanceLongTaskTiming::PerformanceLongTaskTiming(
+    double startTime, double endTime, String frameContextUrl)
+    : PerformanceEntry(frameContextUrl, "longtask", startTime, endTime)
+{
+}
+
+PerformanceLongTaskTiming::~PerformanceLongTaskTiming()
+{
+}
+
+DEFINE_TRACE(PerformanceLongTaskTiming)
+{
+    PerformanceEntry::trace(visitor);
+}
+
+} // namespace blink