Fix bug when tooltip anchor is removed from document without being unmounted

[chandlerprall]

Fixes #1105 by adding a requestAnimationFrame loop when a tooltip is open, each iteration tests if the anchor element is contained by document.

Tested in Chrome, FF, and Safari by linking EUI module into Kibana and testing the ML panel @jgowdyelastic reported issue with.

[snide]

@chandlerprall and I think this is a roundabout solution to the problem because it basically constantly checks while the tooltip is up. We're going to make an issue to revisit it later. We did however want a quick solution for 6.4 that would help the ML team. I can't see any new issues this strategy creates.

I tested this in IE11, Firefox and chrome in as many tooltip usages (context menus, focused inputs, icontips...etc) as I could think of. Along with checking ML, I also tested by manually destroying the anchor through inspect / and debug tools. It seems to solve the issue.

I think this is a big enough issue for ML that we're going to release a new build of EUI in 6.4 with it.