Use Traversal<HTMLElement> API in HTMLTableRowsCollection

Use Traversal<HTMLElement> API in HTMLTableRowsCollection for clarity and
performance:
- It makes rowAfter() and lastRow() more consistent
- It makes it clear we are only interested in HTMLElements
- We can use hadLocalName() instead of hasTagName() as we only iterate through
  HTML elements
- We can use the slightly faster lastChild(const ContainerNode&) overload in
  lastRow() now that we know at compile time the argument is not merely a
  generic Node.

R=esprehn, adamk
BUG=346733

Review URL: https://codereview.chromium.org/206743005

git-svn-id: svn://svn.chromium.org/blink/trunk@170120 bbb929c8-8fbe-4397-9dbb-9b2b20218538