Makes Window::GetTargetBounds() work with mixed layerless windows

BUG=none
TEST=covered by test
R=ben@chromium.org

Review URL: https://codereview.chromium.org/98073007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239408 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239408