[FragmentItem] Make delta_to_next_for_same_layout_object_ mutable

This member needs to be set in |LayoutBox::AddLayoutResult|,
after |NGFragmentItems| was built and set to the cached list.

Since this member is used as a cached index, allow changing
by marking it as |mutable|.

This change is needed to properly mark |NGFragmentItem| as
|const|.

This patch has no behavior changes.

Bug: 982194
Change-Id: I83131b744cd73994b861be9eb374e5e5e47fd5c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2138998Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756990}

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

@@ -435,7 +435,7 @@ void NGFragmentItem::RecalcInkOverflow(
   }
 }
 
-void NGFragmentItem::SetDeltaToNextForSameLayoutObject(wtf_size_t delta) {
+void NGFragmentItem::SetDeltaToNextForSameLayoutObject(wtf_size_t delta) const {
   DCHECK_NE(delta, 0u);
   delta_to_next_for_same_layout_object_ = delta;
 }

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

@@ -129,7 +129,7 @@ class CORE_EXPORT NGFragmentItem : public RefCounted<NGFragmentItem>,
   wtf_size_t DeltaToNextForSameLayoutObject() const {
     return delta_to_next_for_same_layout_object_;
   }
-  void SetDeltaToNextForSameLayoutObject(wtf_size_t delta);
+  void SetDeltaToNextForSameLayoutObject(wtf_size_t delta) const;
 
   const PhysicalRect& RectInContainerBlock() const { return rect_; }
   const PhysicalOffset& OffsetInContainerBlock() const { return rect_.offset; }
@@ -359,7 +359,7 @@ class CORE_EXPORT NGFragmentItem : public RefCounted<NGFragmentItem>,
   std::unique_ptr<NGInkOverflow> ink_overflow_;
 
   // Item index delta to the next item for the same |LayoutObject|.
-  wtf_size_t delta_to_next_for_same_layout_object_ = 0;
+  mutable wtf_size_t delta_to_next_for_same_layout_object_ = 0;
 
   // Note: We should not add |bidi_level_| because it is used only for layout.
   unsigned type_ : 2;           // ItemType