paper-tooltip does not automatically add a 'describedby'

[lukeschaefer]

The aria spec recommends that "elements with the role tooltip are referenced through the use of aria-describedby before or at the time the tooltip is displayed."

The Google Chrome a11y dev tools test suite has a rule for enforcing this.

This means that in order to make the application conform to a11y best practices when having a tooltip, you have to add aria-described for each element with a tooltip:

<paper-radio-button name="2" aria-describedby="fair">
       <paper-tooltip id='fair'>Fair</paper-tooltip>
       <iron-icon icon="star"></iron-icon>
</paper-radio-button>

If paper-tooltip has an id, then it should have enough to be able to do this automatically:

this.target.setAttribute('aria-describedby', this.id)