Here's the game plan:

- Launch customized built-in elements,  Issue 652579 
- Make document.register print a deprecation message
- When this use counter is low enough, remove document.register <https://www.chromestatus.com/metrics/feature/timeline/popularity/457>

We can't add a deprecation message yet because there's no path forward for people using "is" but there should be soon. Some people have expressed discomfort with deprecation messages if we're not on a clear path to removal. This is fuzzier but I think document.register use has peaked and will only decline further now that customElements.define has reached stable.

Sounds like a plan to me! I also think once Polymer 2 is solidified and in
the wild, devs will quickly be brought over to using CE v1.

Bulk disowning per sshruthi's email about bug triage best practices.

Let me merge this to deprecating CEv0.

As is often pointed out by API owners, unless the usage is quite low,
devs will learn *ignoring* deprecation message.