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As part of FTP-048, unions will be migrated to use explicit
ordinals. This updates all FIDL unions to use the explicit ordinal
syntax.

Bug: fuchsia:40859
Change-Id: Ie4cab2101cd7340affe8148cad7989a86776bbcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899910Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Felix Zhu <fcz@google.com>
Cr-Commit-Position: refs/heads/master@{#714009}