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As this is the last usage of Content Service by Assistant, the Content
Service-related APIs on AssistantController/AssistantViewDelegate are
also cleaned up in this CL.

Bug: b:146351046
Change-Id: I852a17630e3d9e3b17b8dfac025648d693004dc4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006228Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#732936}