[LayoutNG] Fieldset legend fragmentation

Add fragmentation support for fieldset legends.

Unit tests added to NGFieldsetLayoutAlgorithmTest.

The following work remains in order for legend fragmentation to be
considered complete and will be addressed in follow-up changes:

1. Making sure the fieldset border starts at the correct offset during
fragmentation when a legend is added.
2. Web tests will need to be added to test #1, as well as other
potential fragmentation scenarios.
3. Get fragmentation working in the case of a large border and small
legend, and add a unit test for this.
4. Refactor to reduce the amount of code in the Layout() function.
5. Handle fieldset content with a negative margin border start
correctly.

Bug: 875235
Change-Id: Idf1947af4ca9b26892bc6fc76e72cc1bbec4afa6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090762
Commit-Queue: Alison Maher <almaher@microsoft.com>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749368}

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

@@ -30,7 +30,9 @@ class CORE_EXPORT NGFieldsetLayoutAlgorithm
  private:
   const NGConstraintSpace CreateConstraintSpaceForLegend(
       NGBlockNode legend,
-      LogicalSize available_size);
+      LogicalSize available_size,
+      LogicalSize percentage_size,
+      LayoutUnit block_offset);
   const NGConstraintSpace CreateConstraintSpaceForFieldsetContent(
       NGBlockNode fieldset_content,
       LogicalSize padding_box_size,