[LayoutNG] Write back spanner offset to the legacy object.

It may be the wrong year to do this, but fixing 22 tests with such a
tiny effort still seems worth it. :)

Bug: 829028
Change-Id: I5e34fc18edcb063d125933aac9174ff08a5acc40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860421
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706125}

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

@@ -920,8 +920,7 @@ void NGBlockNode::PlaceChildrenInFlowThread(
   for (const auto& child : physical_fragment.Children()) {
     if (child->GetLayoutObject() != box_) {
       DCHECK(child->GetLayoutObject()->IsColumnSpanAll());
-      // TODO(mstensho): Write back the spanner offset to the associated
-      // LayoutMultiColumnSpannerPlaceholder (if we bother)
+      CopyChildFragmentPosition(*child, child.offset);
       continue;
     }
     // Each anonymous child of a multicol container constitutes one column.