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This API can be used from both ash and browser tests, and exposes
handlers to access, inspect and use the Embedded Assistant UI.

In this CL I also update the Assistant code to use a specific set of
view IDs. This makes it easier to find a specific view during the
unittests (and removes the need for the |GetDescendantViewByName|
methods that I introduced earlier).

BUG: b/141264108
Change-Id: Ib50bae7be1f14c4ce3aabbb31b8a5689b6300619
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1873291
Commit-Queue: Jeroen Dhollander <jeroendh@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709158}