GN: is_skia_standalone

is_skia_standalone will be defined and set to true iff we're using our
standalone BUILDCONFIG.gn.  Any other build (Fuchsia, Chrome, Pdfium)
will have their own BUILDCONFIG.gn, which presumably does not define
is_skia_standalone.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2357963002

Review-Url: https://codereview.chromium.org/2357963002
2 files changed