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

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235267 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235267