DCHECK marker end offset in DocumentMarkerController.

This patch introduces a DCHECK checking if DocumentMarker.endoffset is
not over text length when adding each marker.

Bug: 868229
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I4f2681460d1de1aacf78b55d7bba454dfc557cf2
Reviewed-on: https://chromium-review.googlesource.com/1149763
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579307}

third_party/blink/renderer/core/editing/markers/document_marker_controller.cc

@@ -271,6 +271,7 @@ void DocumentMarkerController::AddMarkerInternal(
 
 void DocumentMarkerController::AddMarkerToNode(const Node& node,
                                                DocumentMarker* new_marker) {
+  DCHECK_GE(ToText(node).length(), new_marker->EndOffset());
   possibly_existing_marker_types_ = possibly_existing_marker_types_.Add(
       DocumentMarker::MarkerTypes(new_marker->GetType()));
   SetContext(document_);