GN: fontmgr_custom

This makes skia_use_fontconfig=false build on Linux.  Instead of using
fontconfig it looks in /usr/share/fonts.

That alone is a nice feature to have, but it's mostly relevant for MSAN,
where we can't link against any uninstrumented system libraries.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2318133003

Review-Url: https://codereview.chromium.org/2318133003
1 file changed