Crash when inner and outer spanning cells end indexes are same.

Position of the cells below spanning cells is not correct So Assert is failing.

When inner and outer spanning cells end indexes are same than lower rows
positions was updating based on changed height by outer spanning cells
but total changed height should be changed height by (inner spanning cell
 + outer spanning cell).

R=jchaffraix@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21909002

git-svn-id: svn://svn.chromium.org/blink/trunk@155581 bbb929c8-8fbe-4397-9dbb-9b2b20218538