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With this patch, scripts can use a 'prompt' action to ask questions
instead of ending the script. The UI of 'prompt' looks just like the UI
that appears between scripts, so the choice between a prompt and
multiple scripts should be a scripter implementation decision.

The prompt action, can:
 - allow a subset of the page to be touchable, as controlled by the
   focus action, like what happens between scripts
 - propose suggestions to be displayed in chips, to simulate script
   execution
 - react to element existence, to simulate preconditions

Limitation: Prompt does not allow navigation outside of the current
page, nor page reloads.

Bug: 1340309
Change-Id: I033399621e5afa0d02021cac6cab6765c913970c
Reviewed-on: https://chromium-review.googlesource.com/c/1343266
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610114}