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

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

Index: third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.h
diff --git a/third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.h b/third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.h
index ac43b1cac556039399b1a9f872c02d89f9c2e22e..780f7a0c3e692932d4565461c2b2ab499a5c6b7c 100644
--- a/third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.h
+++ b/third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.h
@@ -105,7 +105,14 @@ class CORE_EXPORT NGLineBreaker {
   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_;
 };