Fixes bug that resulted in status bubble getting hidden

Interesting enough this was happening because of a focus bug. Turns
out a FocusController associated with one DesktopNativeWidgetAura
could attempt to focus an aura::Window in a different
DesktopNativeWidgetAura. When this happened it would move the
content_window on top of the status bar.

We should only activate windows within the same root.

BUG=345698
TEST=covered by unit test, see bug as well.
R=ben@chromium.org

Review URL: https://codereview.chromium.org/186733011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255714 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255714