SkPDF: unify drawText and drawPosText

Motivation: a later CL will add drawTextBlob() (after
https://crrev.com/2084533004 lands).  This CL is designed
with that change in mind.  Also fewer redundant lines of
code.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2241683005

Review-Url: https://codereview.chromium.org/2241683005