SkPDF: PDFDevice::ContentEntry now implemented with SinglyLinkedList

add SkSinglyLinkedList<T> class

no change in SkPDF output.

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1839633003

Review URL: https://codereview.chromium.org/1839633003
4 files changed