[custom-elements]: `ShadowRoot.prototype.innerHTML` isn't patched.

[bicknellr]

This issue was previously tracked in https://github.com/webcomponents/custom-elements/issues/73, but got closed when the repo was archived after the polyfill moved here.

[stale[bot]]

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.