Mac: Disconnect DesktopMediaPicker window from the parent before closing

The window wasn't removing itself from the parent's children list, which
causes linked bug with some versions of Cocoa.

BUG=331058

Review URL: https://codereview.chromium.org/125133006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243212 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243212