[skia] Explicitly disable XPS backend (flutter#177050)

Skia's working on some breaking changes to the XPS backend. Flutter does
not use this backend explicitly, but it is being compiled in on Windows
builds. We can explicitly disable it all the time to avoid any
unexpected breakages from impacting Flutter.

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Author: kjlubick

engine/src/flutter/tools/gn

@@ -485,6 +485,7 @@ def to_gn_args(args):
   gn_args['skia_use_expat'] = True
   gn_args['skia_use_fontconfig'] = args.enable_fontconfig
   gn_args['skia_use_icu'] = True
+  gn_args['skia_use_xps'] = False  # Flutter does not use XPS
   gn_args['is_official_build'] = True  # Disable Skia test utilities.
   gn_args['android_full_debug'] = args.target_os == 'android' and args.unoptimized
   if args.clang is None: