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This change updates the protocol used to control the PromptAction to
make it more powerful and more in-line with the current protocol:

 - the suggestion and element existence results are merged into one list
 of choice.

 - the string result is replaced by an opaque server_payload result,
 which allows the server to include whatever data it wants.

To make that work, this changes forwards the data proto as an opaque
byte array between layers instead of a single UTF8 string that was used
for both message and result.

Bug: 1343266
Change-Id: I601c31504bd231ef9904bced92fc350a939b6f9d
Reviewed-on: https://chromium-review.googlesource.com/c/1346461
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610576}