Don't destroy the window in PlatformWindow's destructor.
This should have already been closed before we get to the destructor. Otherwise we get double deletes since the Destroy() call can end up calling the delegate which will delete this object.
Also in the X11 implementation, don't call delegate's OnClosed() before null'ing xwindow_, otherwise Destroy() can be called multiple times.
This is split off https://codereview.chromium.org/
1139673003
BUG=484234
Review URL: https://codereview.chromium.org/
1132313003
Cr-Commit-Position: refs/heads/master@{#330381}