Get scroll pos in a more portable way.

For #605. React gets scroll this way-- https://github.com/facebook/react/blob/5d3b12bb3bd6a092cf00ede07b8255a8399c2e58/src/vendor/core/dom/getUnboundedScrollPosition.js

I have confirmed this going Back restores scroll pos in safari firefox chrome... also in IE11, though you see the page scrolling back to its old position.