Oilpan: Element factories should return PassRefPtrWillBeRawPtr<>.

This CL replaces reutrn types of all of Element factory functions witn
PassRefPtrWillBeRawPtr<>.

Also, this introduces DEFINE_NODE_FACTORY macro. We're going to change content
of factories again, and DEFINE_NODE_FACTORY will makes it easier.

BUG=357163

Review URL: https://codereview.chromium.org/279883003

git-svn-id: svn://svn.chromium.org/blink/trunk@173815 bbb929c8-8fbe-4397-9dbb-9b2b20218538