gyp/sfntly: set SK_SFNTLY_SUBSETTER uniformly

When Skia is built into the Android framework,
SK_SFNTLY_SUBSETTER="sample/chromium/font_subsetter.h".
This sets the same value for Skia's test framework

I will eventually move away from the
   #include PREPROCESSOR_DEFINE
trick, which does not work everywhere.

No public API changes.
TBR=reed@google.com

Review-Url: https://codereview.chromium.org/2274223002
2 files changed