Clean up LayoutTableSection::calcRowLogicalHeight() a bit.

Since nobody apparently wants to deal with a rowspanned cell unless we're at
its first row, just skip that cell for subsequent rows, instead of having
checks for this everywhere.

Also removed a debug hashmap that was just used to assert that we didn't add
duplicate cells to a vector. Check the vector directly instead.

Review-Url: https://chromiumcodereview.appspot.com/2434033003
Cr-Commit-Position: refs/heads/master@{#426555}