Remove findRoot function, use Node::highestAncestor instead.

They are equivalent.

BUG=none
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/145493009

git-svn-id: svn://svn.chromium.org/blink/trunk@165695 bbb929c8-8fbe-4397-9dbb-9b2b20218538