Avoid some code duplication in HTMLTableRowsCollection::rowAfter()

Avoid some code duplication in HTMLTableRowsCollection::rowAfter() by
factoring out the code for finding a table row element among children
to a new function. This code was repeated 3 times.

R=adamk, eseidel

Review URL: https://codereview.chromium.org/166093004

git-svn-id: svn://svn.chromium.org/blink/trunk@167242 bbb929c8-8fbe-4397-9dbb-9b2b20218538