Add CompositionMarkerList in preparation for DocumentMarkerController refactor

third_party/WebKit/Source/core/editing/markers/CompositionMarkerList.cpp

+// Copyright 2017 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/editing/markers/CompositionMarkerList.h"
+
+namespace blink {
+
+CompositionMarkerList::CompositionMarkerList() : EditingMarkerList() {}
+
+DocumentMarker::MarkerType CompositionMarkerList::allowedMarkerType() const {
+  return DocumentMarker::Composition;
+}
+
+}  // namespace blink