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

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

+// 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.
+
+#ifndef PerformanceLongTaskTiming_h
+#define PerformanceLongTaskTiming_h
+
+#include "core/timing/PerformanceEntry.h"
+#include "platform/heap/Handle.h"
+#include "wtf/Forward.h"
+#include "wtf/text/WTFString.h"
+
+namespace blink {
+
+class Document;
+
+class PerformanceLongTaskTiming final : public PerformanceEntry {
+    DEFINE_WRAPPERTYPEINFO();
+public:
+    static PerformanceLongTaskTiming* create(double startTime, double endTime, String frameContextUrl)
+    {
+        return new PerformanceLongTaskTiming(startTime, endTime, frameContextUrl);
+    }
+
+    DECLARE_VIRTUAL_TRACE();
+
+private:
+    PerformanceLongTaskTiming(double startTime, double endTime, String frameContextUrl);
+    ~PerformanceLongTaskTiming() override;
+};
+
+} // namespace blink
+
+#endif // PerformanceLongTaskTiming_h