Use PostLayoutChildren in NGPhysicalLineBoxFragment

Another fix to use |PostLayoutChildren|, this time in
|NGPhysicalLineBoxFragment::ScrollableOverflow|.

DevTools console hits the DCHECK added in r795416
<crrev.com/c/2339699>, this patch fixes the case.

Bug: 965639
Change-Id: Ia3a74eb45c17923fa879dd0b663f48b441481968
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2345941Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Auto-Submit: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796733}

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

@@ -96,7 +96,7 @@ PhysicalRect NGPhysicalLineBoxFragment::ScrollableOverflow(
     TextHeightType height_type) const {
   const WritingMode container_writing_mode = container_style.GetWritingMode();
   PhysicalRect overflow;
-  for (const auto& child : Children()) {
+  for (const auto& child : PostLayoutChildren()) {
     PhysicalRect child_scroll_overflow =
         child->ScrollableOverflowForPropagation(container, height_type);
     child_scroll_overflow.offset += child.Offset();