This is the v0 branch of CustomElements from the webcomponents.js project.
It is necessary to have polymer run within a content script
Install via npm:
npm install webcomponentsjs-custom-element-v0
Within a content script (see enable-webcomponents-in-content-scripts and habitlab for usage examples)
document.registerElement = null
require('webcomponentsjs-custom-element-v0')
This is taken from the 0.7.24 release of the webcomponents.js npm package.