Row with only spanning cells have a height of 0.

While distributing extra height of the row spanning cell into spanning rows,
Rows are not getting any height those contains only spanning cells because these
rows already having zero height.

Now, We are assigning one part of spanning cell's height based on their span
value to the rows those contains only spanning cells.

R=jchaffraix@chromium.org
BUG=252694

Review URL: https://chromiumcodereview.appspot.com/23530044

git-svn-id: svn://svn.chromium.org/blink/trunk@158038 bbb929c8-8fbe-4397-9dbb-9b2b20218538