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

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

Index: third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.cc
diff --git a/third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.cc b/third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.cc
index 739502db352f383b49857406ec378f5fe31193b3..6a04416468b1171f0de214bf450cdf85d4f5c0ce 100644
--- a/third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.cc
+++ b/third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.cc
@@ -457,6 +457,10 @@ void NGLineBreaker::HandleOpenTag(const NGInlineItem& item,
                                  borders.inline_start + paddings.inline_start;
       position_ += item_result->inline_size;
 
+      // While the spec defines "non-zero margins, padding, or borders" prevents
+      // line boxes to be zero-height, tests indicate that only inline direction
+      // of them do so. See should_create_line_box_.
+      // Force to create a box, because such inline boxes affect line heights.
       item_result->needs_box_when_empty =
           item_result->inline_size || item_result->margins.inline_start;
       if (item_result->needs_box_when_empty && !should_create_line_box_)