Option to toggle ChromeVox if installed

[peterh-capella]

Within the "Text" panel of the accessibility content we get the very helpful content of what a screen reader might read. Seeing the handy "speaking-person" icon I expected to be able to click this and hear what would be read by the screen reader. Disappointed I thought it would be an excellent feature to have this icon actually toggle the enabled state of ChromeVox if the user has that extension also installed. If that seems to be over-loading this control, having a link to do so would be useful as the text content is only a part of what screen readers announce, so being able to hear what they actually announce would be useful for developers that don't normally check.