Check NGPaintFragment::AssociateWithLayoutObject() to handle valid next for same layout object list

This patch introduces |DCHECK()|s to |AssociateWithLayoutObject()| to help
identify root cause of issue 1039470.

Bug: 1039470
Change-Id: I29aeb05eb7015c62f9908d9ce538460d72742aa2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2004176
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732315}

third_party/blink/renderer/core/paint/ng/ng_paint_fragment.cc

@@ -506,12 +506,15 @@ void NGPaintFragment::AssociateWithLayoutObject(
       return;
     }
     // This |layout_object| was fragmented across multiple blocks.
+    DCHECK_EQ(layout_object, first_fragment->GetLayoutObject());
     NGPaintFragment* last_fragment = first_fragment->LastForSameLayoutObject();
     last_fragment->next_for_same_layout_object_ = this;
     return;
   }
-  DCHECK(add_result.stored_value->value);
-  add_result.stored_value->value->next_for_same_layout_object_ = this;
+  NGPaintFragment* last_fragment = add_result.stored_value->value;
+  DCHECK(last_fragment) << layout_object;
+  DCHECK_EQ(layout_object, last_fragment->GetLayoutObject());
+  last_fragment->next_for_same_layout_object_ = this;
   add_result.stored_value->value = this;
 }