Move some properties from "required" to "supported" on scrollbar role

[melanierichards]

Addresses #858

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[jnurthen]

link for review

[carmacleod]

This sentence:

Authors MUST set the aria-valuemin, aria-valuemax, and aria-valuenow attributes.

should probably just be:

Authors MUST set the value of the aria-valuenow attribute.

The list that follows the above sentence may need rewording as well:

If missing, their implicit values follow the same rules as the HTML range input type:

• If aria-valuemin is missing or not a number, it defaults to 0 (zero).
• If aria-valuemax is missing or not a number, it defaults to 100.
• If aria-valuenow is missing or not a number, it defaults to the value half way between aria-valuemin and aria-valuemax.
• If aria-valuenow is present but less than aria-valuemin, it defaults to the value of aria-valuemin.
• If aria-valuenow is present but greater than aria-valuemax, it defaults to the value of aria-valuemax.

[melanierichards]

Methinks I was a little too literal in addressing the issues for these and missed some prose update opportunities, thanks @carmacleod for all your feedback! :)

Made some changes to the prose, left repair techniques in there for now because Core-AAM doesn't seem to cover the overflow repair techniques (where valuenow is less than valuemin, for example). I imagine this may warrant further discussion, especially given that, from a quick test, the Windows browsers aren't following these repair techniques: https://jsfiddle.net/r39sqne2/2/

[melanierichards]

Should be up to date now based on 2019-05-23 call.