Use DEFINE_ELEMENT_TYPE_CASTS*() macros for Element subclasses

Use DEFINE_ELEMENT_TYPE_CASTS*() macros for Element subclasses instead of
the DEFINE_NODE_TYPE_CASTS*() ones. This makes the new Traversal<> API work
for these Element types as well.

R=eseidel, adamk
BUG=346733

Review URL: https://codereview.chromium.org/190633010

git-svn-id: svn://svn.chromium.org/blink/trunk@168797 bbb929c8-8fbe-4397-9dbb-9b2b20218538