GN: add some missing SkCodec defines

third_party/libwebp/BUILD.gn

Index: third_party/libwebp/BUILD.gn
diff --git a/third_party/libwebp/BUILD.gn b/third_party/libwebp/BUILD.gn
index d27004bdf4490452fe995eea8a32d0d4b58c49a7..efac7d2e095ed1d8b93ef444a6689fb27a66fe11 100644
--- a/third_party/libwebp/BUILD.gn
+++ b/third_party/libwebp/BUILD.gn
@@ -11,6 +11,14 @@ import("../third_party.gni")
 third_party("libwebp") {
   public_include_dirs = [ "../externals/libwebp/src" ]
 
+  defines = [
+    # WebP naturally decodes to RGB_565, and we work with BGR_565.
+    # This makes WebP decode to BGR_565 when we ask for RGB_565.
+    # (It also swaps the color order for 4444, but we don't care today.)
+    # TODO: swizzle ourself in SkWebpCodec instead of requiring this non-standard libwebp.
+    "WEBP_SWAP_16BIT_CSP",
+  ]
+
   sources = [
     "../externals/libwebp/src/dec/alpha.c",
     "../externals/libwebp/src/dec/buffer.c",