CustomElementUpgradeSorter puts elements in shadow-including tree order.

This is part of "element definition", step 21.
https://html.spec.whatwg.org/multipage/scripting.html#dom-customelementsregistry-define

BUG=594918

Review-Url: https://codereview.chromium.org/1995933005
Cr-Commit-Position: refs/heads/master@{#395014}
4 files changed