Allow reactions to enqueue other reactions for the same element

When upgrade reactions[1] upgrade elements[2], it may enqueue callback
reactions.

This patch allows CustomElementReactionStack to enqueue() for the same
element from within popInvokingReactions().

[1] https://html.spec.whatwg.org/multipage/scripting.html#upgrade-reaction
[2] https://html.spec.whatwg.org/multipage/scripting.html#upgrades

BUG=594918

Review-Url: https://codereview.chromium.org/2052283002
Cr-Commit-Position: refs/heads/master@{#399480}
3 files changed