When a view is recycled, Select to Speak reading stops immediately. This is the correct behavior in non-ARC because we don't want to read nodes after the pages they are from are closed. But it's not right in ARC, where recycler views cause things to pop in and out all the time. Repro: Read some gmail overview with STS, then scroll it offscreen. Reading stop suddenly.

Possible solutions:
- STS should cache text content within a scrollable. (note the existence of AutomationNode.prototype.scrollable)
This pattern is currently only used in ARC++, but could easily be translated to web and other tree sources
- AXTreeSourceArc could retain offscreen nodes, keeping their data, but potentially not their parent links. This would be something like ref counting in the extension renderer and would likely require some additional fleshing out