[LayoutNG] Disallow hit-testing NG list markers

List markers are not selectalbe. Before this change, it still
hits in |LayoutNGMixin::PositionForPoint()| as atomic inline,
which leads to crashes.

Change-Id: I4563c493417cf72900f9da48a052f564fb5f0369
Bug: 964813, 964814, 964925
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619593
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661376}

third_party/blink/renderer/core/layout/layout_inline.h

@@ -358,7 +358,7 @@ class CORE_EXPORT LayoutInline : public LayoutBoxModelObject {
       TransformState&,
       VisualRectFlags = kDefaultVisualRectFlags) const final;
 
-  PositionWithAffinity PositionForPoint(const LayoutPoint&) const final;
+  PositionWithAffinity PositionForPoint(const LayoutPoint&) const override;
 
   IntRect BorderBoundingBox() const final {
     IntRect bounding_box = EnclosingIntRect(PhysicalLinesBoundingBox());

third_party/blink/renderer/core/layout/ng/layout_ng_mixin.h

@@ -48,7 +48,7 @@ class LayoutNGMixin : public Base {
                    const LayoutPoint& accumulated_offset,
                    HitTestAction) final;
 
-  PositionWithAffinity PositionForPoint(const LayoutPoint&) const final;
+  PositionWithAffinity PositionForPoint(const LayoutPoint&) const override;
 
   NGPaintFragment* PaintFragment() const final {
     // TODO(layout-dev) crbug.com/963103

third_party/blink/renderer/core/layout/ng/list/layout_ng_inside_list_marker.cc

@@ -24,4 +24,9 @@ bool LayoutNGInsideListMarker::IsOfType(LayoutObjectType type) const {
          LayoutInline::IsOfType(type);
 }
 
+PositionWithAffinity LayoutNGInsideListMarker::PositionForPoint(
+    const LayoutPoint&) const {
+  return CreatePositionWithAffinity(0);
+}
+
 }  // namespace blink

third_party/blink/renderer/core/layout/ng/list/layout_ng_inside_list_marker.h

@@ -30,6 +30,7 @@ class CORE_EXPORT LayoutNGInsideListMarker final : public LayoutInline {
 
  private:
   bool IsOfType(LayoutObjectType) const override;
+  PositionWithAffinity PositionForPoint(const LayoutPoint&) const override;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutNGInsideListMarker,

third_party/blink/renderer/core/layout/ng/list/layout_ng_list_marker.cc

@@ -53,4 +53,9 @@ bool LayoutNGListMarker::NeedsOccupyWholeLine() const {
   return false;
 }
 
+PositionWithAffinity LayoutNGListMarker::PositionForPoint(
+    const LayoutPoint&) const {
+  return CreatePositionWithAffinity(0);
+}
+
 }  // namespace blink

third_party/blink/renderer/core/layout/ng/list/layout_ng_list_marker.h

@@ -32,6 +32,7 @@ class CORE_EXPORT LayoutNGListMarker final
 
  private:
   bool IsOfType(LayoutObjectType) const override;
+  PositionWithAffinity PositionForPoint(const LayoutPoint&) const override;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutNGListMarker, IsLayoutNGListMarker());