[LayoutNG] Follow up on CL:1607719

This patch addresses review comment on crrev.com/c/1607719
by adding more comments.

No behavior changes.

Change-Id: I1362fd4dd18467eeb0e30aecb0e80c7cf9bd43cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609718Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659133}

third_party/blink/renderer/core/layout/ng/inline/ng_inline_box_state.cc

@@ -435,7 +435,8 @@ unsigned NGInlineLayoutStateStack::UpdateBoxDataFragmentRange(
       box_data_list_[box_data_index - 1].fragmented_box_data_index =
           box_data_list_.size();
       // Do not use `emplace_back()` here because adding to |box_data_list_| may
-      // reallocate the buffer. Create a new instance and |push_back()| instead.
+      // reallocate the buffer, but the `BoxData` ctor must run before the
+      // reallocation. Create a new instance and |push_back()| instead.
       BoxData fragmented_box_data(box_data_list_[box_data_index - 1],
                                   start_index, index);
       box_data_list_.push_back(fragmented_box_data);