[LayoutNG] Refactor NGLineBreaker for ShapingLineBreaker

third_party/WebKit/Source/core/layout/ng/inline/ng_inline_break_token.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/ng/inline/ng_inline_break_token.h"
 
 #include "core/layout/ng/inline/ng_inline_node.h"
 
 namespace blink {
 
 NGInlineBreakToken::NGInlineBreakToken(NGInlineNode* node,
                                        unsigned item_index,
                                        unsigned text_offset)
     : NGBreakToken(kInlineBreakToken, kUnfinished, node),
       item_index_(item_index),
-      text_offset_(text_offset) {}
+      text_offset_(text_offset) {
+  // Use nullptr for the initial layout, rather than (0, 0) break token.
+  DCHECK(item_index || text_offset);
+}
 
 NGInlineBreakToken::NGInlineBreakToken(NGLayoutInputNode* node)
     : NGBreakToken(kInlineBreakToken, kFinished, node),
       item_index_(0),
       text_offset_(0) {}
 
 }  // namespace blink