SkPDF: Add Sfntly to DEPS, gyp

Note: this can be disabled via:
    GYP_DEFINES='skia_pdf_use_sfntly=0

Warning: dm is 34% slower and uses 9% more memory.  This is
okay.

Motivation: We want to test this code path in DM, since it is
always used by Chromium and Android.

BUG=skia:3563

Committed: https://skia.googlesource.com/skia/+/6a53b04e26749ea61f690ece408f2a1c0a5ad5bb

Review URL: https://codereview.chromium.org/1134683006
5 files changed