Revert "Remove SetCanFocus from the window server"

This reverts commit 655fc905aed0ff73f8b510ba1b4fcb9c5faaf474.

Reason for revert: It looks like I'm going to need this to fix 928939

Original change's description:
> Remove SetCanFocus from the window server
> 
> I've noticed that this SetCanFocus() method has been there for
> a while but nobody is using it. I believe this is not used anymore.
> 
> Bug: none
> Test: none
> Change-Id: Ifb041c696ce4c08a6fc0b45bea35ee3b6c3516f9
> Reviewed-on: https://chromium-review.googlesource.com/c/1447315
> Reviewed-by: Scott Violet <sky@chromium.org>
> Reviewed-by: Tom Sepez <tsepez@chromium.org>
> Commit-Queue: Jun Mukai <mukai@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#629072}

TBR=mukai@chromium.org,sky@chromium.org,tsepez@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: none
Change-Id: I6ca42ca22d321303d1089d4d36a8e046b7c441a2
Reviewed-on: https://chromium-review.googlesource.com/c/1456682
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629662}
12 files changed