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Before this change, a PromptProto.Choice with only element_exists would
be rendered as a chip with no names.

After this change, we can add PromptProto.Choice again with only
element_exists set and no chip definitions set, neither name, nor icon,
nor chip type. This is useful to script automatic end of the prompt
action.

Bug: b/132950869
Change-Id: Ie552934311d8345c4a5496b10d7a63e2bc97c705
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617520Reviewed-by: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Commit-Queue: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Auto-Submit: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662549}