[DMC #24] Add SpellingMarker and GrammarMarker (subclasses of DocumentMarker)

third_party/WebKit/Source/core/editing/markers/GrammarMarker.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/GrammarMarker.h"
+
+namespace blink {
+
+GrammarMarker::GrammarMarker(unsigned start_offset,
+                             unsigned end_offset,
+                             const String& description)
+    : SpellCheckMarker(DocumentMarker::kGrammar,
+                       start_offset,
+                       end_offset,
+                       description) {
+  DCHECK_LT(start_offset, end_offset);
+}
+
+}  // namespace blink