Add temporary staging GN arg for Skia roll (#41743)

In order to enable the new behavior, dl_imagefilter_unittests need to be
updated to reflect new bounds mapping values. This requires an atomic PR
to disable the legacy arg and update after skia rolls.

Required to land https://skia-review.googlesource.com/c/skia/+/693058
and progress on https://skbug.com/9283

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Author: lhkbob

tools/gn

@@ -302,6 +302,7 @@ def to_gn_args(args):
   gn_args['skia_use_fontconfig'] = args.enable_fontconfig
   gn_args['skia_use_legacy_layer_bounds'
          ] = True  # Temporary: See skbug.com/12083, skbug.com/12303.
+  gn_args['skia_use_legacy_colorfilter_imagefilter'] = True  # Temporary staging
   gn_args['skia_use_icu'] = True
   gn_args['is_official_build'] = True  # Disable Skia test utilities.
   gn_args['android_full_debug'