Revert 239232 "Make sure WindowObservers are removed from window..."

> Make sure WindowObservers are removed from window before destruction
> 
> * Check if the window observer is empty upon destruction.
> * remove the all observers in ~WindowObserver()
> * Remove RemoveObserver that are no longer necessary
> * Fix shell shutdown order so that observers are removed correctly before deleting all windows.
> 
> BUG=324018
> R=skuhne@chromium.org, sky@chromium.org
> 
> Review URL: https://codereview.chromium.org/101013002

TBR=oshima@chromium.org

Review URL: https://codereview.chromium.org/103863003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239239 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239239