Add TrustedTypes support

[rictic]

Trusted Types is launching in Chrome soon, and it would be great if Polymer core supported it.

[shhnjk]

It'd be really great if there is a Trusted Types support! Chromium uses Polymer intensively and it is hard to deploy Trusted Types to Chromium browser internal pages (aka WebUI) without this. See https://bugs.chromium.org/p/chromium/issues/detail?id=1098690.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[freshp86]

@rictic Is this change published into any NPM version yet? We would need that published so that we can use it in Chromium to address https://crbug.com/1098690.

[rictic]

@bicknellr Do you know when we last did an npm release of Polymer?

[bicknellr]

https://www.npmjs.com/package/@polymer/polymer claims it was about two years ago.

[rictic]

@freshp86 tracking this work in #5699