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In Chromium build the libassistant might be unavailable. In this case we
want to hide the Assistant opt-in flow.

Bug: b/141330938
Test: Manual Test
Change-Id: I6796cab496892938a7c026e1a72d1fc4a4b25296
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1817319
Commit-Queue: Yue Li <updowndota@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699877}