Add backup element queue to CustomElementReactionStack

This patch adds a backup element queue[1], which is to be added to the
spec in PR[2].

The attached test not only fail but also crashes without this change.

[1] https://html.spec.whatwg.org/multipage/scripting.html#backup-element-queue
[2] https://github.com/whatwg/html/pull/1412

BUG=594918

Review-Url: https://codereview.chromium.org/2097463002
Cr-Commit-Position: refs/heads/master@{#402776}
9 files changed