Fix Assert failure for test case bug7714.html

While distributing spanning cell's extra height in rows, some spanning cells
does not have enough extra height to distribute in that case, last row's
height of the previous spanning cell is updated more then once so position
of the row are not proper and below row gets negative height. To stop
updating row's height twice, row's height sets back to its original position.

R=jchaffraix@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22415003

git-svn-id: svn://svn.chromium.org/blink/trunk@155825 bbb929c8-8fbe-4397-9dbb-9b2b20218538