[LayoutNG] Add comments about "certain zero-height line boxes"

third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NGLineBreaker_h
 #define NGLineBreaker_h
 
 #include "core/CoreExport.h"
 #include "core/layout/ng/inline/ng_inline_item_result.h"
 #include "core/layout/ng/inline/ng_inline_node.h"
@@ skipping 87 matching lines @@
   unsigned item_index_;
   unsigned offset_;
   LayoutUnit position_;
   WTF::Optional<NGLayoutOpportunity> opportunity_;
   NGLogicalOffset content_offset_;
   LazyLineBreakIterator break_iterator_;
   HarfBuzzShaper shaper_;
   ShapeResultSpacing<String> spacing_;
 
   bool auto_wrap_;
+
+  // We don't create "certain zero-height line boxes".
+  // https://drafts.csswg.org/css2/visuren.html#phantom-line-box
+  // Such line boxes do not prevent two margins being "adjoining", and thus
+  // collapsing.
+  // https://drafts.csswg.org/css2/box.html#collapsing-margins
   bool should_create_line_box_;
+
   bool is_after_forced_break_;
 };
 
 }  // namespace blink
 
 #endif  // NGLineBreaker_h