Implement WKWebView to replace WebView

Summary:
@public

`UIWebView` has been deprecated and replaced by `WKWebView`. This diff introduces a new component `WKWebView` that simply renders a `WKWebView` on iOS.

This is the first in the stack of many diffs that'll be required to fully replace `UIWebView` with `WKWebView` in the `<WebView/>` React Native component. Eventually, I hope to introduce a prop called `useWebKitImplementation`, which, when true, will force RN to use `WKWebView` instead of `UIWebView` for the `<WebView/>` component.

The only thing that's been implemented so far is the `source` property.

Reviewed By: mmmulani

Differential Revision: D6266100

fbshipit-source-id: 65862e34bd98db7fff0349cf26888afee43a56e4

Libraries/Components/WKWebView/WKWebView.android.js

@@ -0,0 +1,22 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ * @flow
+ * @providesModule WKWebView
+ */
+
+const React = require('React');
+const View = require('View');
+const Text = require('Text');
+
+module.exports = () => {
+  return (
+    <View>
+      <Text>Android version not implemented.</Text>
+    </View>
+  );
+};