Replace WebCore::highestAncestor(), in htmledigint.cpp, by Node::highestAncestor()

This patch replaces WebCore::highestAncestor() by Node::highestAncesotr() to reduce code size since both implementations are identical.

BUG=n/a
TEST=n/a; No behavior changes.
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/23499010

git-svn-id: svn://svn.chromium.org/blink/trunk@157277 bbb929c8-8fbe-4397-9dbb-9b2b20218538