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This CL attempts to make minimal changes required to:
- Remove standalone Assistant UI classes from code base
- Clean up remnants of standalone UI from AssistantUiController.

Note that additional clean up work will be done after:
- Removing no longer needed visibility states (e.g. hidden)
- Removing no longer needed UI modes (e.g. main, mini, and web)
- Removing no longer needed caption bar class

Bug: b:148080975
Change-Id: I9ac1ac5caa9399dfd8b5463a0238221d4b8e9bb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032416
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737569}