[FragmentItem] Fix DCHECK failures for virtual/layout_ng_block_frag/

Fixes DCHECK failures when NGFragmenttItem is enabled for:
virtual/layout_ng_block_frag/external/wpt/css/css-break/widows-orphans-005.html

There are 120 other DCHECK failures at the same stack,
probably fixes them too.

Bug: 829028, 982194
Change-Id: I1ed7257d72cd43ddc5a1f3901f5ff89189a247ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942596Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720308}

third_party/blink/renderer/core/layout/ng/ng_physical_fragment.cc

@@ -424,11 +424,11 @@ const Vector<NGInlineItem>& NGPhysicalFragment::InlineItemsOfContainingBlock()
     const {
   DCHECK(IsInline());
   DCHECK(GetLayoutObject());
-  LayoutBlockFlow* block_flow = GetLayoutObject()->ContainingNGBlockFlow();
+  LayoutBlockFlow* block_flow =
+      GetLayoutObject()->RootInlineFormattingContext();
   // TODO(xiaochengh): Code below is copied from ng_offset_mapping.cc with
   // modification. Unify them.
   DCHECK(block_flow);
-  DCHECK(block_flow->ChildrenInline());
   NGBlockNode block_node = NGBlockNode(block_flow);
   DCHECK(block_node.CanUseNewLayout());
   NGLayoutInputNode node = block_node.FirstChild();