GN: add some missing SkCodec defines

third_party/libwebp/BUILD.gn

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
 }
 
 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",
     "../externals/libwebp/src/dec/frame.c",
     "../externals/libwebp/src/dec/idec.c",
     "../externals/libwebp/src/dec/io.c",
     "../externals/libwebp/src/dec/quant.c",
     "../externals/libwebp/src/dec/tree.c",
     "../externals/libwebp/src/dec/vp8.c",
     "../externals/libwebp/src/dec/vp8l.c",
@@ skipping 56 matching lines @@
     "../externals/libwebp/src/utils/huffman.c",
     "../externals/libwebp/src/utils/huffman_encode.c",
     "../externals/libwebp/src/utils/quant_levels.c",
     "../externals/libwebp/src/utils/quant_levels_dec.c",
     "../externals/libwebp/src/utils/random.c",
     "../externals/libwebp/src/utils/rescaler.c",
     "../externals/libwebp/src/utils/thread.c",
     "../externals/libwebp/src/utils/utils.c",
   ]
 }