[layoutng] CalculateDefaultBlockSize can return too small values

It needs to return at least border_scrollbar_padding_, or else
this can lead to a DCHECK failure in ng_length_utils.cc

Found by running ScrollbarsTest.AutosizeAlmostRemovableScrollbar with
LayoutNG enabled.

R=ikilpatrick@chromium.org, mstensho@chromium.org

Change-Id: I9635ef98274bf573151756d19b3879fb6d33080a
Reviewed-on: https://chromium-review.googlesource.com/c/1330348Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607065}

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

@@ -371,15 +371,15 @@ NGLogicalOffset NGBlockLayoutAlgorithm::CalculateLogicalOffset(
 scoped_refptr<NGLayoutResult> NGBlockLayoutAlgorithm::Layout() {
   NGBoxStrut borders = ComputeBorders(ConstraintSpace(), Node());
   NGBoxStrut padding = ComputePadding(ConstraintSpace(), Style());
-
   border_padding_ = borders + padding;
-  NGLogicalSize border_box_size = CalculateBorderBoxSize(
-      ConstraintSpace(), Node(), CalculateDefaultBlockSize(), border_padding_);
 
   NGBoxStrut scrollbars = Node().GetScrollbarSizes();
   border_scrollbar_padding_ = ConstraintSpace().IsAnonymous()
                                   ? NGBoxStrut()
                                   : border_padding_ + scrollbars;
+  NGLogicalSize border_box_size = CalculateBorderBoxSize(
+      ConstraintSpace(), Node(), CalculateDefaultBlockSize(), border_padding_);
+
   child_available_size_ =
       ShrinkAvailableSize(border_box_size, border_scrollbar_padding_);
 
@@ -2299,7 +2299,8 @@ LayoutUnit NGBlockLayoutAlgorithm::CalculateDefaultBlockSize() {
   if (Node().IsQuirkyAndFillsViewport()) {
     LayoutUnit block_size = ConstraintSpace().AvailableSize().block_size;
     block_size -= ComputeMarginsForSelf(ConstraintSpace(), Style()).BlockSum();
-    return block_size.ClampNegativeToZero();
+    return std::max(block_size.ClampNegativeToZero(),
+                    border_scrollbar_padding_.BlockSum());
   }
   return NGSizeIndefinite;
 }