Revert of Revert of ui::PlatformWindowDelegate::OnClosed() isn't called when a ui...

Revert of Revert of ui::PlatformWindowDelegate::OnClosed() isn't called when a ui::X11Window is closed (patchset #1 id:1 of https://codereview.chromium.org/647573002/)

Reason for revert:
This was not the cause of the regression. See http://build.chromium.org/p/chromium.memory/builders/Linux%20ASan%20LSan%20Tests%20%281%29/builds/6569

Original issue's description:
> Revert of ui::PlatformWindowDelegate::OnClosed() isn't called when a ui::X11Window is closed (patchset #1 id:1 of https://codereview.chromium.org/644443002/)
>
> Reason for revert:
> Only cl that seems related to http://build.chromium.org/p/chromium.memory/builders/Linux%20ASan%20LSan%20Tests%20%281%29/builds/6561 falures.
>
> Original issue's description:
> > ui::PlatformWindowDelegate::OnClosed() isn't called when a ui::X11Window is closed
> >
> > This problem manifests itself in simple Mojo apps that use NativeViewportService on X11. When the NativeViewport window is closed, the NativeViewportClient OnDestroyed() method does not run.
> >
> > BUG=421536
> >
> > Committed: https://crrev.com/b37606e1c2a6703e58a61a978e8e32be72acae0c
> > Cr-Commit-Position: refs/heads/master@{#298926}
>
> TBR=ben@chromium.org,hansmuller@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=421536
>
> Committed: https://crrev.com/b93c31988a55a9cb2a328e39520f56788585b542
> Cr-Commit-Position: refs/heads/master@{#298985}

TBR=ben@chromium.org,hansmuller@chromium.org,jwd@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=421536

Review URL: https://codereview.chromium.org/642953003

Cr-Commit-Position: refs/heads/master@{#298999}