SkAdvancedTypefaceMetrics: abstract out linked list

src/core/SkSinglyLinkedList.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #ifndef SkSinglyLinkedList_DEFINED
 #define SkSinglyLinkedList_DEFINED
 
 #include <utility>
@@ skipping 13 matching lines @@
             Node* next = node->fNext;
             SkASSERT(next != nullptr || node == fTail);
             delete node;
             node = next;
         }
         fHead = nullptr;
         fTail = nullptr;
     }
     T* back() { return fTail ? &fTail->fData : nullptr; }
     T* front() { return fHead ? &fHead->fData : nullptr; }
+    bool empty() const { return fHead == nullptr; }
     #ifdef SK_DEBUG
     int count() {  // O(n), debug only.
         int count = 0;
         for (Node* node = fHead; node; node = node->fNext) {
             ++count;
         }
         return count;
     }
     #endif
     void pop_front() {
@@ skipping 43 matching lines @@
         Node* fNext;
         template <class... Args>
         Node(Args&&... args) : fData(std::forward<Args>(args)...), fNext(nullptr) {}
     };
     Node* fHead;
     Node* fTail;
     SkSinglyLinkedList(const SkSinglyLinkedList<T>&) = delete;
     SkSinglyLinkedList& operator=(const SkSinglyLinkedList<T>&) = delete;
 };
 #endif  // SkSinglyLinkedList_DEFINED