Use getActiveElement module

We had something that did the same sort of protection. The module
differs slightly (returns document.body instead of undefined) but
looking at the callers, that should be ok.

src/core/ReactInputSelection.js

@@ -20,17 +20,9 @@
 
 var ReactDOMSelection = require('ReactDOMSelection');
 
+var getActiveElement = require('getActiveElement');
 var nodeContains = require('nodeContains');
 
-// It is not safe to read the document.activeElement property in IE if there's
-// nothing focused.
-function getActiveElement() {
-  try {
-    return document.activeElement;
-  } catch (e) {
-  }
-}
-
 function isInDocument(node) {
   return nodeContains(document.documentElement, node);
 }