Use Traversal<>::firstAncestor() API more in the code base

Use Traversal<>::firstAncestor() API more in the code base to simplify the
code and increase readability.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/476273004

git-svn-id: svn://svn.chromium.org/blink/trunk@180398 bbb929c8-8fbe-4397-9dbb-9b2b20218538