Summary: Rows that were virtualized and currently not rendered weren't copied, because we were using the DOM to get their content. This changes this to use out `textContent` utility to get the value of a row and therefore enables copying rows that are not rendered.
Reviewed By: jknoxville
Differential Revision: D15898935
fbshipit-source-id: c372bc4e77477214860f0513b442374da8f72416
8bc26378f0