Optimize number of rowspan cells required to distribute their height in rows.

Rowspan cell duplicates were present in rowSpanCells list when when rowspan
cell have collspan value more than 1.

Now, rowspan cell added only once in the rowSpanCells list.

R=jchaffraix@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23526051

git-svn-id: svn://svn.chromium.org/blink/trunk@157891 bbb929c8-8fbe-4397-9dbb-9b2b20218538