X11 and Ozone: Move closing logic to DWTHPlatform

Close and CloseNow were move to DWTHPlatform. The same was done with the
container of children DWTH.

Bug: 990756
Change-Id: Ic09852c3a22e780738d98da3f4fdfee4a6741a52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1771407
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694665}