This canaries the idea by converting gyp/pdf.gypi to gn/pdf.gni.
If this lands and rolls quietly, I'll flesh out the rest.
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3972
Change-Id: Iecaf1ccce7891679904c585946272bfb25eea299
Reviewed-on: https://skia-review.googlesource.com/3972
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
