Register for processing Custom Elements at microtask checkpoint time.

r166252 added a work queue that's processed at a microtask
checkpoint. Make Custom Elements also use that queue.

BUG=

Review URL: https://codereview.chromium.org/143903022

git-svn-id: svn://svn.chromium.org/blink/trunk@166499 bbb929c8-8fbe-4397-9dbb-9b2b20218538