Replace custom code by hasOneTextChild API

This code just is the hasOneTextChild logic, so
to avoid confusion use that instead.

Review URL: https://codereview.chromium.org/692153002

git-svn-id: svn://svn.chromium.org/blink/trunk@184843 bbb929c8-8fbe-4397-9dbb-9b2b20218538