Use new Traversal<HTML*Element> API more in accessibility

Use new Traversal<HTML*Element> API more in accessibility. This makes the code
a bit shorter / simpler.

R=adamk, eseidel
BUG=346733

Review URL: https://codereview.chromium.org/189463011

git-svn-id: svn://svn.chromium.org/blink/trunk@168793 bbb929c8-8fbe-4397-9dbb-9b2b20218538