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Styling still WIP. This change enables the UI to show a prompt chip
when a composite command is selected.
- Selecting a composite command clears the input and shows a chip with
a prompt
- Backspace over the prompt when the input is empty informs the backend
that the prompt was cancelled, restores the previous user input, and
requeries

Bug: 1014639
Change-Id: I3fff2bf6cacab28b473c8ddee3a7a41fbb6df945
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530796Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826338}