SkAdvancedTypefaceMetrics: abstract out linked list

src/core/SkSinglyLinkedList.h

Index: src/core/SkSinglyLinkedList.h
diff --git a/src/pdf/SkSinglyLinkedList.h b/src/core/SkSinglyLinkedList.h
similarity index 98%
rename from src/pdf/SkSinglyLinkedList.h
rename to src/core/SkSinglyLinkedList.h
index c7101141d6421b1b0cbb2691e56002bc99a1f3fb..b01120461378e3d6c132aa7a93d72211b77de7b4 100644
--- a/src/pdf/SkSinglyLinkedList.h
+++ b/src/core/SkSinglyLinkedList.h
@@ -31,6 +31,7 @@ public:
     }
     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;