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This creates a common place for current and future use_wuffs_xxx_parser
GN args.

Change-Id: I8e08522cab23a7bded2206f04cda916cca450b82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2299688Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789407}