Add DocumentMarkerList interface

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

Index: third_party/WebKit/Source/core/editing/markers/DocumentMarkerList.cpp
diff --git a/third_party/WebKit/Source/core/editing/markers/DocumentMarkerList.cpp b/third_party/WebKit/Source/core/editing/markers/DocumentMarkerList.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..c13e4e7ebc3ea57b4b420152db4f5bcc8fbc58ea
--- /dev/null
+++ b/third_party/WebKit/Source/core/editing/markers/DocumentMarkerList.cpp
@@ -0,0 +1,13 @@
+// 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/DocumentMarkerList.h"
+
+namespace blink {
+
+DocumentMarkerList::DocumentMarkerList() = default;
+
+DocumentMarkerList::~DocumentMarkerList() = default;
+
+}  // namespace blink