fix(content): attach the right onNativeSelectionChange. (#2150)

This undoes a recent change https://github.com/ianstormtaylor/slate/pull/2131

Which hooked up `this.handlers.onNativeSelectionChange` to `'selectionchange'` as well as `this.handlers.onBeforeInput` to `'beforeinput'`.

It turns out that in order to fix the iOS issues, we only needed the change to `onBeforeInput`. The corect handler for `'selectionchange'` is the instance method `this.onNativeSelectionChange`. This broke a whole bunch of focus related issues.

packages/slate-react/src/components/content.js

@@ -91,7 +91,7 @@ class Content extends React.Component {
 
     window.document.addEventListener(
       'selectionchange',
-      this.handlers.onNativeSelectionChange
+      this.onNativeSelectionChange
     )
 
     // COMPAT: Restrict scope of `beforeinput` to clients that support the
@@ -113,7 +113,7 @@ class Content extends React.Component {
     if (window) {
       window.document.removeEventListener(
         'selectionchange',
-        this.handlers.onNativeSelectionChange
+        this.onNativeSelectionChange
       )
     }