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This is a reland of a230653d

There is a global singleton in user_activity which is used from
multiple components - therefore, ui/base/user_activity itself has to
be a component.

Original change's description:
> Add BUILD.gn for ui/base/user_activity
>
> ui/base/user_activity causes ui/base to have a dependency on ui/events.
> To remove another instance of this dependency, put user_activity into
> its own component.
>
>
> Bug: 921723
> Change-Id: I8c82bcb82b876386c63fa263937280c48ceb43fb
> Reviewed-on: https://chromium-review.googlesource.com/c/1448735
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Daniel Libby <dlibby@microsoft.com>
> Cr-Commit-Position: refs/heads/master@{#628473}

Bug: 921723
Change-Id: If80874d9cf2ae032d8931bfa3aa292b63bfa7119
Reviewed-on: https://chromium-review.googlesource.com/c/1452996Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#629140}