[FragmentItem] Fix AsDisplayItemClient

This patch chagnes |AsDisplayItemClient| to return
|DisplayItemClient|.

This is to prepare for making |NGFragmentItem| not a
|DisplayItemClient|.

There should be no behavior changes.

Bug: 982194
Change-Id: I2361b0c316b3625ee85004b42480ca37e980d8f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208840
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770500}

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

@@ -55,8 +55,9 @@ Color SelectionBackgroundColor(const Document& document,
 
 // TODO(yosin): Remove |AsDisplayItemClient| once the transition to
 // |NGFragmentItem| is done. http://crbug.com/982194
-inline const NGFragmentItem& AsDisplayItemClient(const NGInlineCursor& cursor) {
-  return *cursor.CurrentItem();
+inline const DisplayItemClient& AsDisplayItemClient(
+    const NGInlineCursor& cursor) {
+  return *cursor.Current().GetDisplayItemClient();
 }
 
 inline const NGPaintFragment& AsDisplayItemClient(