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Two methods in AssistantClient interface is actually for the
communication between ash and the browser (and the comment also
said so). This CL replaces those two methods by C++ along with
removing the comment.

- OpenAssistantSettings(); this can be replaced as a single
  function.
- SetDeviceActions(); this comes up with the idea of having an
  interface for holding browser-owned objects which are used by Ash.
  This is the superclass of AssistantClient so it's named so.
  Since AssistantImageDownloder and AssistantSetup are owned by
  AssistantClient, this CL makes the new ash::AssistantSetup own
  them. With that, the static GetInstance()s are removed.

TBR=stevenjb@chromium.org

Bug: 958193
Test: trybot
Change-Id: I2d308e1d59777b952409f0c2c9bda1392d9b5154
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1638179
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665659}