Introduce HasHTMLTagName functor and use it with ElementTraversal

Introduce HasHTMLTagName functor and use it with ElementTraversal to simplify
the code a bit.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/471173003

git-svn-id: svn://svn.chromium.org/blink/trunk@180363 bbb929c8-8fbe-4397-9dbb-9b2b20218538