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This plumbing provides activation and focusing of the WindowTreeHost's
child windows, enabling full use of the mouse and keyboard in
content views.

Also fixed some IWYU linter errors.

Bug: 878439
Change-Id: I0fc344b69edf78ca3aff39f615951cfa016718b2
Reviewed-on: https://chromium-review.googlesource.com/1194868
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587686}