GN: make libwebp an optional dependency

This will be handy for folks who don't have libwebp, like Fuchsia.

I convinced myself that this is done right by:
  - building in all three modes (default and explicitly set both ways);
  - looking at verbose Ninja logs to see the presence/lack of SK_HAS_WEBP_LIBRARY;
  - running dm -m Codec, which passes with libwebp and segfault without it.

If this is viable, I intend to make all third-party dependencies optional
and follow this pattern.  :skia should link and degrade gracefully without
any of //third_party.  It's okay for tools to have hard third-party
dependencies; we just need them to get past the `gn gen` stage without them.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2270693004

Review-Url: https://codereview.chromium.org/2270693004

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