To support magnifying glass on GeckoView, I would like to add `dragcaret`
event and, clientX and clientX in CaretStateChangedEvent chrome event.

Actually, accessible caret fires `presscaret` and `releasecaret` when
accessbile caret is pressed or released. But when dragging this caret, no
chrome event is fired. Since magnifying glass listens to moving this caret,
I would like `dargcaret` for GeckoView.

Also, Users' dragging point is necessary to set better position of magnifying
glass windows. So I also want client point of dragging point on `presscaret`
and `dragcaret` event.

This event and properties are on layout.accessiblecaret.magnifier.enabled=true,
So this can be only for GeckoView.

Differential Revision: https://phabricator.services.mozilla.com/D137965