Revert 235267 "Fix crash in WebContentsViewAura::WindowObserver ..."

> Fix crash in WebContentsViewAura::WindowObserver that happens when switching tabs. The exact repro steps were:
> 
> 1) open an empty page
> 2) print preview
> 3) open a new tab
> 4) go back to original tab
> 5) close print preview
> 6) switch to the new tab
> 
> The crash happened at step 4 because the second tab got removed from the same root window as the print preview's observer. We weren't unobserving all the children of the root window at that point.
> 
> BUG=318791
> R=ben@chromium.org
> 
> Review URL: https://codereview.chromium.org/69813005

TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/63013004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235277 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235277