[pdf] Prepend subset tag to BaseFont and FontName

PDF 32000-1:2008 Section 9.6.4 FontSubsets
"The tag shall consist of six uppercase letters" "followed by a plus
sign" "different subsets in the same PDF file shall have different

Since Skia always tries to subset fonts in PDF, always prepend a subset

Bug: skia:12581
Change-Id: I6d62d3abc801238226dd01f6dd299fef78c05137
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/476457
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
3 files changed