Make AssociateNextForSameLayoutObject() not to hit DCHECK for list marker

This patch changes |NGFragmentItemsBuilder::AssociateNextForSameLayoutObject()|
not to hit DCHECK about the first item in |NGFragmentItems| for list marker.

This is follow-up of the CL[1] introduced wrong DCHECK().

[1] http://crrev.com/c/1930370 [FragmentItem] Introduce next for same layout
object cache in NGFragmentItem

Bug: 982194
Change-Id: Ic37559dafb795c34529f72fef2c8dff95814094e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1943651
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720132}

third_party/blink/renderer/core/layout/ng/inline/ng_fragment_items_builder.cc

@@ -198,7 +198,10 @@ void NGFragmentItemsBuilder::ToFragmentItems(WritingMode writing_mode,
 }
 
 void NGFragmentItemsBuilder::AssociateNextForSameLayoutObject() {
-  DCHECK(items_.IsEmpty() || items_[0]->Type() == NGFragmentItem::kLine);
+  // items_[0] can be:
+  //  - kBox  for list marker, e.g. <li>abc</li>
+  //  - kLine for line, e.g. <div>abc</div>
+  DCHECK(items_.IsEmpty() || items_[0]->IsContainer()) << items_[0];
   HashMap<const LayoutObject*, wtf_size_t> last_fragment_map;
   for (wtf_size_t index = 1u; index < items_.size(); ++index) {
     const NGFragmentItem& item = *items_[index];

third_party/blink/renderer/core/layout/ng/inline/ng_inline_cursor_test.cc

@@ -431,6 +431,13 @@ TEST_P(NGInlineCursorTest, NextWithEllipsis) {
   EXPECT_THAT(list, ElementsAre("#linebox", "abcdefghi", "abcd", u"#'\u2026'"));
 }
 
+TEST_P(NGInlineCursorTest, NextWithListItem) {
+  NGInlineCursor cursor = SetupCursor("<ul><li id=root>abc</li></ul>");
+  Vector<String> list = ToDebugStringList(cursor);
+  EXPECT_THAT(list,
+              ElementsAre("LayoutNGListMarker (anonymous)", "#linebox", "abc"));
+}
+
 TEST_P(NGInlineCursorTest, NextWithSoftHyphens) {
   // Use "Ahem" font to get U+2010 as soft hyphen instead of U+002D
   LoadAhem();