Consider slots' tabindex in focus navigation

third_party/WebKit/Source/core/html/HTMLSlotElement.h

 /*
  * Copyright (C) 2015 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 50 matching lines @@
     bool hasSlotChangeEventListener();
 
     void updateDistributedNodesWithFallback();
     void didUpdateDistribution();
 
     void attach(const AttachContext& = AttachContext()) final;
     void detach(const AttachContext& = AttachContext()) final;
 
     void attributeChanged(const QualifiedName&, const AtomicString& oldValue, const AtomicString& newValue, AttributeModificationReason = ModifiedDirectly) final;
 
+    short tabIndex() const override;
+
     DECLARE_VIRTUAL_TRACE();
 
 private:
     HTMLSlotElement(Document&);
 
     enum DistributionState {
         DistributionOnGoing,
         DistributionDone,
         DistributionChanged,
         DistributionUnchanged
@@ skipping 12 matching lines @@
     WillBeHeapVector<RefPtrWillBeMember<Node>> m_distributedNodes;
     WillBeHeapHashMap<RawPtrWillBeMember<const Node>, size_t> m_distributedIndices;
     // TODO(hayato): Remove m_oldDistibutedNodes and make SlotAssignment check the diffirence between old and new distributed nodes for each slot to save the memories.
     WillBeHeapVector<RefPtrWillBeMember<Node>> m_oldDistributedNodes;
     DistributionState m_distributionState;
 };
 
 } // namespace blink
 
 #endif // HTMLSlotElement_h