[layoutng] Only set paint fragment on inline nodes

Addresses kojii@'s comment on https://crrev.com/c/1001721

R=kojii@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I3632cc61d4eb957d1cff67bbdd41cd9cbe0d0f75
Reviewed-on: https://chromium-review.googlesource.com/1005095Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549501}

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

@@ -163,12 +163,15 @@ scoped_refptr<NGLayoutResult> NGBlockNode::Layout(
   }
   LayoutBlockFlow* block_flow =
       box_->IsLayoutNGMixin() ? ToLayoutBlockFlow(box_) : nullptr;
+  NGLayoutInputNode first_child = FirstChild();
   scoped_refptr<NGLayoutResult> layout_result;
   if (box_->IsLayoutNGMixin()) {
     layout_result = ToLayoutBlockFlow(box_)->CachedLayoutResult(
         constraint_space, break_token);
     if (layout_result) {
-      block_flow->SetPaintFragment(layout_result->PhysicalFragment());
+      block_flow->ClearPaintFragment();
+      if (first_child && first_child.IsInline())
+        block_flow->SetPaintFragment(layout_result->PhysicalFragment());
       return layout_result;
     }
   }
@@ -185,7 +188,6 @@ scoped_refptr<NGLayoutResult> NGBlockNode::Layout(
       layout_result->UnpositionedFloats().IsEmpty()) {
     DCHECK(layout_result->PhysicalFragment());
 
-    NGLayoutInputNode first_child = FirstChild();
     if (block_flow && first_child && first_child.IsInline()) {
         CopyFragmentDataToLayoutBoxForInlineChildren(
             ToNGPhysicalBoxFragment(*layout_result->PhysicalFragment()));