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Latest revision to PRD specifies:
* Rename section to "Assistant"
* Remove the search engine link to browser settings
* Hide the whole section when assistant isn't available

See bug for screenshots.

This CL plumbs "showAssistant" from settings UI to main to page
similarly to how showApps is handled.

TODO: Once we're sure there won't be other search settings here,
rename the files/section to "os_assistant_page".

Bug: 960937
Test: manual on device (with assistant) and on linux-chromeos (without)
Change-Id: I97e4409f6751123bad2ce364230adb17e7d7e9ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1650525Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667371}