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I had centralized this in DesktopNativeWidgetAura, but it needs to be
in the DesktopRootWindowHost implementations as it may be invoked
during construction so that state can be set up. Otherwise we may
crash if during init state isn't set up.

I added test for coverage.

BUG=313780
TEST=covered by unit test
R=ben@chromium.org

Review URL: https://codereview.chromium.org/49303007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233012 0039d316-1c4b-4281-b951-d872f2087c98