SkPDF: hand SfntlyWrapper::SubsetFont() ttcIndex, not fontName.

Also, minor code refactoring.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2258233002

Committed: https://skia.googlesource.com/skia/+/fce190647285423bf36c44bca09db78a6af30f9f
Review-Url: https://codereview.chromium.org/2258233002
1 file changed