Remove Internals::createContentElement.

We don't need Internals::createContentElement anymore.
Just document.createElement('content') works fine.

BUG=337728

Review URL: https://codereview.chromium.org/155703003

git-svn-id: svn://svn.chromium.org/blink/trunk@166815 bbb929c8-8fbe-4397-9dbb-9b2b20218538