MathML: Allow a fragmentainer break *before* an mrow.

While we don't expect to do block fragmentation *inside* an mrow (it's
monolithic), it's perfectly fine to break *before* it. Correct a DCHECK
to allow incoming IsBreakBefore() break tokens.

Fixes external/wpt/mathml/relations/css-styling/display-2.html when
LayoutNGBlockFragmentation is enabled.

Bug: 829028
Change-Id: Idb0e03f99112f32b3d86430dd97a88e6cb14db14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3178984
Reviewed-by: Ian Kilpatrick <[email protected]>
Commit-Queue: Morten Stenshorne <[email protected]>
Cr-Commit-Position: refs/heads/main@{#924309}

Author: mstensho

third_party/blink/renderer/core/layout/ng/mathml/ng_math_row_layout_algorithm.cc

@@ -8,6 +8,7 @@
 #include "third_party/blink/renderer/core/layout/ng/mathml/ng_math_layout_utils.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_block_break_token.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_box_fragment.h"
+#include "third_party/blink/renderer/core/layout/ng/ng_fragmentation_utils.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_length_utils.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_out_of_flow_layout_part.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_physical_box_fragment.h"
@@ -183,7 +184,7 @@ void NGMathRowLayoutAlgorithm::LayoutRowItems(
 }
 
 scoped_refptr<const NGLayoutResult> NGMathRowLayoutAlgorithm::Layout() {
-  DCHECK(!BreakToken());
+  DCHECK(!IsResumingLayout(BreakToken()));
 
   bool is_display_block_math =
       Node().IsMathRoot() && (Style().Display() == EDisplay::kBlockMath);