[FragmentItem] Add missing declarations

It compiles without this because the superclass
|DisplayItemClient| has the declarations.

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

There should be no behavior changes.

Bug: 982194
Change-Id: I3f908131e9de418f2910591fb7d52984ee6dda27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208879
Auto-Submit: Koji Ishii <kojii@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770490}

third_party/blink/renderer/core/layout/ng/inline/ng_fragment_item.h

@@ -407,6 +407,9 @@ inline bool NGFragmentItem::CanReuse() const {
   return false;
 }
 
+CORE_EXPORT std::ostream& operator<<(std::ostream&, const NGFragmentItem*);
+CORE_EXPORT std::ostream& operator<<(std::ostream&, const NGFragmentItem&);
+
 }  // namespace blink
 
 #endif  // THIRD_PARTY_BLINK_RENDERER_CORE_LAYOUT_NG_INLINE_NG_FRAGMENT_ITEM_H_