Makes ~RootWindow delete child windows

This way any children of the Root can call GetRootWindow() from their
destructor and get a valid root.

BUG=307369
TEST=covered by unit test
R=ben@chromium.org

Review URL: https://codereview.chromium.org/27570004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229586 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229586