[LayoutNG] Instroduce const version of NGPaintFragment::LastForSameLayoutObject()

This patch introduces const version of |NGPaintFragment::
LastForSameLayoutObject()| to avoid callers to use |const_cast| for improving
code health.

This patch is a preparation of the patch[1].

[1] http://crrev.com/c/1341776 [LayoutNG] Implement LayoutNG version of
{First,Last}CharacterAfterWhitespaceCollapsing()

Change-Id: I2dfad3861db1e93b89f5ca2441c2e222bee0181d
Reviewed-on: https://chromium-review.googlesource.com/c/1341775
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609261}

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

@@ -501,6 +501,10 @@ NGPaintFragment::FragmentRange NGPaintFragment::InlineFragmentsFor(
   return FragmentRange(nullptr, false);
 }
 
+const NGPaintFragment* NGPaintFragment::LastForSameLayoutObject() const {
+  return const_cast<NGPaintFragment*>(this)->LastForSameLayoutObject();
+}
+
 NGPaintFragment* NGPaintFragment::LastForSameLayoutObject() {
   NGPaintFragment* fragment = this;
   while (fragment->next_for_same_layout_object_)

third_party/blink/renderer/core/paint/ng/ng_paint_fragment.h

@@ -272,6 +272,7 @@ class CORE_EXPORT NGPaintFragment : public RefCounted<NGPaintFragment>,
   // for a LayoutObject.
   static FragmentRange InlineFragmentsFor(const LayoutObject*);
 
+  const NGPaintFragment* LastForSameLayoutObject() const;
   NGPaintFragment* LastForSameLayoutObject();
 
   // Called when lines containing |child| is dirty.