[FlexNG] Don't provide a baseline when an element has layout containment.

In https://chromium-review.googlesource.com/c/chromium/src/+/1988376
I added a TODO (which was bad, and actually a note to myself).

With FlexNG enabled the lack of this behaviour triggers a lot failures:
https://test-results.appspot.com/data/layout_results/linux-rel/292399/webkit_layout_tests%20%28with%20patch%29/layout-test-results/results.html

This makes it so an element with layout containment doesn't provide
a baseline to its parent.

An alternate version of this patch would to add this logic into
NGBoxFragment::Baseline. However this would likely be slightly more
expensive as we invoke that function much more than creating a fragment.

Bug: 845235
Change-Id: I2faa5042881c14e1e90088ac0219a0aa556f2bf3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2019103Reviewed-by: David Grogan <dgrogan@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735572}

third_party/blink/renderer/core/layout/ng/ng_box_fragment.h

@@ -31,7 +31,6 @@ class CORE_EXPORT NGBoxFragment final : public NGFragment {
     if (GetWritingMode() != physical_fragment_.Style().GetWritingMode())
       return base::nullopt;
 
-    // TODO containment?
     return To<NGPhysicalBoxFragment>(physical_fragment_).Baseline();
   }
 

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

@@ -83,6 +83,7 @@ NGPhysicalBoxFragment::NGPhysicalBoxFragment(
               ? kFragmentRenderedLegend
               : kFragmentBox,
           builder->BoxType()) {
+  DCHECK(layout_object_);
   DCHECK(layout_object_->IsBoxModelObject());
   if (NGFragmentItemsBuilder* items_builder = builder->ItemsBuilder()) {
     has_fragment_items_ = true;
@@ -108,10 +109,15 @@ NGPhysicalBoxFragment::NGPhysicalBoxFragment(
   is_painted_atomically_ =
       builder->space_ && builder->space_->IsPaintedAtomically();
   border_edge_ = builder->border_edges_.ToPhysical(builder->GetWritingMode());
-  children_inline_ =
-      builder->layout_object_ && builder->layout_object_->ChildrenInline();
-  has_baseline_ = builder->baseline_.has_value();
-  baseline_ = builder->baseline_.value_or(LayoutUnit::Min());
+  children_inline_ = layout_object_->ChildrenInline();
+  if (builder->baseline_.has_value() &&
+      !layout_object_->ShouldApplyLayoutContainment()) {
+    has_baseline_ = true;
+    baseline_ = *builder->baseline_;
+  } else {
+    has_baseline_ = false;
+    baseline_ = LayoutUnit::Min();
+  }
 }
 
 scoped_refptr<const NGLayoutResult>