Reorder CustomElement creation code to come after normal element creation

esprehn saw this code lighting up a bit during profiling. I don't have a perf test that
shows meaningful differences on my workstation, but the new code should theoretically
make the most common case (of constructing built-in elements) slightly faster.

The only possible change in behavior here is if the embedder were to try and
override a built-in HTML element. This sounds unproblematic to me, but it's
possible we should codify this somewhere.

No test changes because It's not possible to test the behavioral change with a layout test,
since it's already impossible (by design) to override builtin elements with document.register().

R=dominicc@chromium.org, esprehn@chromium.org

Review URL: https://codereview.chromium.org/77093003

git-svn-id: svn://svn.chromium.org/blink/trunk@162419 bbb929c8-8fbe-4397-9dbb-9b2b20218538