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

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

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=235277